Series: Insulin, Metabolism and Hormonal Modulators - Episode 9: The Thyroid and Energy Metabolism

In the series "Insulin, Metabolism and Hormonal Modulators," the first eight episodes followed the ways insulin, IGF-1, estrogens and drugs that modify estrogen signaling alter nutrient use and endocrine responses. Episode 9 shifts the spotlight to the thyroid, a system that does not store energy or directly transport glucose but sets an important part of the pace at which the body consumes fuel, produces heat and adapts its organs to energetic demand.

Fitness culture often reduces the thyroid to two caricatures: a "slow metabolism" and a "fast metabolism." Real physiology is far more interesting. T4 must be transported, locally converted by deiodinases, moved into cells and converted into a signal through thyroid hormone receptors. The brain, liver, skeletal muscle, adipose tissue and heart can regulate this information differently. Two people with similar serum values can therefore have different metabolic contexts, and the same person can respond differently during caloric restriction, illness, cold exposure or hard training.

This article explains the hypothalamic-pituitary-thyroid axis, TSH, T4, T3, deiodinases, thermogenesis, mitochondria, glucose and lipid metabolism, the relationship with muscle mass and the particular context of strength athletes. Sections dealing with thyroid hormones used as weight-loss tools are strictly educational: no doses, cycles or administration protocols are provided. Excess thyroid hormone can cause thyrotoxicosis, arrhythmias, muscle catabolism, bone loss and other serious complications.

1. The thyroid: a small gland with a huge metabolic reach

The thyroid is an endocrine gland in the lower front of the neck, yet its metabolic influence extends throughout the body. Thyroid hormones modify the rate at which many tissues use oxygen, generate ATP, consume fuels and produce heat. When thyroid function falls or rises substantially, the effects do not remain in the neck; they appear in the heart, skeletal muscle, liver, adipose tissue, nervous system and gastrointestinal tract. For a strength athlete, the thyroid is not a fat-loss switch. It is one of the central regulators that helps determine how energetic processes are coordinated across the entire organism.

2. The hypothalamic-pituitary-thyroid axis

Control begins in the hypothalamus, which releases TRH, continues in the pituitary, which secretes TSH, and reaches the thyroid gland, where TSH stimulates thyroid-hormone synthesis and release. T4 and T3 then exert negative feedback on the pituitary and hypothalamus. The system behaves less like an on-off switch and more like a biological thermostat: signaling is continuously adjusted according to hormone concentrations, energy availability, temperature, illness and other inputs. Understanding this loop explains why TSH is so useful in thyroid assessment and why interpreting one hormone without the rest of the axis can be misleading.

3. How thyroid hormones are made

Thyroid-hormone synthesis requires iodine, thyroglobulin and specialized enzymes. Iodide is taken up by follicular cells, transported into the follicular lumen and used by thyroid peroxidase to iodinate tyrosine residues within thyroglobulin. Coupling of these residues generates the precursors of T4 and T3, which remain stored in colloid until the gland receives a signal to release them. This architecture gives the thyroid a substantial hormone reserve. Acute changes in diet or a single training session therefore do not instantly translate into dramatic changes in circulating thyroid hormone, even though longer physiological stresses can alter the axis.

4. T4 and T3 are not the same molecule

The thyroid secretes predominantly thyroxine, T4, while a large share of circulating T3 is generated by peripheral conversion of T4. T3 binds thyroid-hormone receptors with greater affinity and is generally the more biologically active form. Calling T4 simply “inactive,” however, is too crude. T4 is the major circulating reservoir and the substrate from which tissues can produce T3 locally. The body therefore controls not only how much thyroid hormone is present in blood but also how much active signaling is generated inside a specific tissue. This local control is a major reason thyroid physiology cannot be reduced to one laboratory number.

5. Binding proteins and the free fraction

Most circulating T4 and T3 is bound to proteins such as thyroxine-binding globulin, transthyretin and albumin. Only a very small fraction is free and available for cellular transport. Changes in binding proteins can therefore alter total hormone concentrations without changing actual thyroid function to the same degree. Free T4 and, in selected situations, free T3 provide information that differs from total T4 and T3. These measurements still need to be interpreted with TSH and the clinical context. A single value divorced from binding proteins, medications, illness and pituitary feedback can produce a very confident but very wrong metabolic story.

6. Thyroid hormone must enter the cell

Thyroid hormones do not simply diffuse freely through every cellular membrane. Transporters including MCT8, MCT10 and members of the OATP family contribute to hormone entry and exit from cells. This control matters because two tissues exposed to the same serum T4 concentration can experience different intracellular thyroid signaling. Modern thyroid biology has therefore shifted the question from “how much T3 is in the blood?” toward “how much active thyroid signal reaches the receptor in the relevant tissue?” For metabolism, that distinction is fundamental. Serum measurements describe the circulation, whereas transport, conversion and receptor biology determine what each cell actually experiences.

7. The D1, D2 and D3 deiodinases

Local thyroid-hormone metabolism is controlled by three deiodinases. D1 contributes to peripheral metabolism and to production of part of circulating T3. D2 converts T4 to T3 in tissues such as the brain, pituitary, brown adipose tissue and skeletal muscle, thereby amplifying local signaling. D3 does the opposite: it inactivates T4 and T3 and reduces tissue exposure. These enzymes are selenoproteins and form a fine regulatory system. They explain why similar serum values can generate different biological effects across organs, energy states and illness. Thyroid signaling is therefore customized inside tissues rather than delivered as one uniform whole-body dose.

8. T3 reaches the nuclear receptor

Once T3 enters the cell and is locally available, it acts mainly through the nuclear receptors TRα and TRβ. These receptors interact with DNA and numerous cofactors to alter gene expression. The effect is not merely “a faster metabolism.” It changes cellular programs involving mitochondrial enzymes, fatty-acid oxidation, ion transport, cardiac contractility and thermogenesis. The final response depends on tissue type, receptor distribution, cofactors and physiological state. Thyroid hormone is therefore better understood as a regulator of the metabolic program than as a stimulant of a single pathway. Its actions spread through thousands of downstream processes that have to remain coordinated.

9. Basal metabolic rate and energy expenditure

Thyroid hormones are major determinants of basal metabolic rate. Hyperthyroidism generally increases energy expenditure and heat production, whereas hypothyroidism tends to reduce them. Basal metabolism, however, is not controlled by the thyroid alone. Lean mass, age, sex, organ size, sympathetic activity and energy intake all contribute substantially. A 140-kilogram Strongman can have very high energy requirements with completely normal thyroid tests simply because a large amount of metabolically active tissue must be maintained. Attributing every difference in calorie requirements to thyroid function ignores body size and tissue composition, which are often the larger pieces of the equation.

10. Thermogenesis: why thyroid hormones generate heat

Some chemical energy from nutrients is not converted into mechanical work or stored as ATP; it is dissipated as heat. Thyroid hormones increase thermogenesis through several mechanisms, including faster ATP turnover, greater ion-pump activity, changes in mitochondrial function and interaction with adrenergic signaling. This is why hyperthyroidism can cause heat intolerance and sweating, whereas hypothyroidism often causes cold intolerance. For a large strength athlete, thermogenesis also has a practical consequence. A massive body performing intense work already generates enormous heat, so any condition that further raises heat production increases the burden on circulation, sweating and environmental heat loss.

11. Mitochondria: the energy machinery under thyroid control

T3 influences mitochondrial biogenesis, respiratory proteins and the capacity of tissues to oxidize substrates. It does not create energy from nothing. Instead, it changes the rate and efficiency with which cells convert carbohydrate and fat energy into ATP and heat. In selected tissues, thyroid signaling supports greater oxidative capacity and faster responses to energetic demand. In skeletal muscle this intersects with training because mechanical and metabolic stimuli also alter mitochondrial density and fuel use. The resulting phenotype therefore reflects both endocrine status and training history. A hormone can set metabolic conditions, but it cannot replace the adaptations produced by repeated exercise.

12. The sodium-potassium pump and the invisible cost of keeping cells alive

A significant part of resting energy expenditure is used to maintain ion gradients. Na+/K+-ATPase consumes ATP to move sodium and potassium across cell membranes, and thyroid hormones can increase the expression and activity of this pump in several tissues. This is an excellent example of an invisible energy cost: the body spends calories even while motionless simply to keep cells electrically and chemically functional. Greater ion cycling contributes to the calorigenic effect of thyroid hormones. In muscle and nerve, these gradients are essential for contraction and signal transmission, linking basic cellular maintenance directly to performance physiology.

13. Brown adipose tissue and adaptive thermogenesis

Brown adipose tissue is specialized for heat production. In this tissue, D2 can increase local conversion of T4 to T3, while thyroid signaling cooperates with sympathetic stimulation to support UCP1 expression and thermogenesis. In adults, the amount and activity of brown fat vary with cold exposure, age and body composition. The broader lesson is more important than the absolute quantity of brown fat: the body can regulate thyroid signaling locally to adjust energy expenditure. A tissue does not need a dramatic rise in serum T3 to intensify its thyroid-dependent metabolic program. Local activation can do much of the work.

14. Thyroid hormones and the sympathetic nervous system

Many features of thyroid-hormone excess resemble adrenergic activation: tachycardia, tremor, sweating, anxiety and heat intolerance. Thyroid hormones are not catecholamines, but they can amplify tissue responsiveness to adrenergic signals by changing receptors and downstream cellular machinery. Energy metabolism therefore emerges from a network linking the thyroid, nervous system and peripheral organs. During maximal lifting, adrenaline and noradrenaline are already high. Superimposing hyperthyroidism on that state can turn a perceived “metabolic boost” into substantially greater cardiovascular stress. The relevant outcome is not simply increased calorie turnover but the total physiological load.

15. Glucose metabolism

Thyroid hormones influence intestinal glucose absorption, hepatic glucose production, glycogen turnover and peripheral carbohydrate use. In excess, they can accelerate both entry and exit of glucose from different compartments, creating a more dynamic metabolism rather than necessarily a healthier one. Hyperthyroidism may be associated with impaired glucose tolerance and greater insulin requirements in some contexts. Hypothyroidism can also alter insulin sensitivity and substrate utilization. The relationship between thyroid function and insulin is bidirectional and tissue dependent. This is why an apparently “fast” metabolism can coexist with poor glucose control, while a slower metabolic state can carry its own form of insulin resistance.

16. The liver: where thyroid hormone redirects fuel flow

The liver is highly responsive to T3. Thyroid hormones can stimulate gluconeogenesis, glycogenolysis, fatty-acid oxidation and the expression of enzymes that regulate cholesterol metabolism. In hyperthyroidism, the liver may release more glucose to support increased whole-body energy demand. In hypothyroidism, hepatic metabolic turnover slows and LDL clearance can fall. These effects explain why glucose, triglycerides and cholesterol may change with thyroid status. They do not mean those markers can be inferred from TSH. Each variable should be measured directly because diet, insulin, body composition, genetics and medications can alter the same pathways independently of thyroid function.

17. Thyroid function and insulin sensitivity

Both thyroid-hormone deficiency and excess can disturb glucose homeostasis, although by different mechanisms. Excess hormone can increase hepatic glucose output and fatty-acid flux, placing greater demands on insulin secretion. Deficiency can reduce glucose use and occur alongside weight gain, dyslipidemia and lower activity. Modern research even explores indices of “thyroid-hormone sensitivity” in relation to metabolic disease, although their clinical role remains unsettled. For athletes, the lesson mirrors the earlier insulin episodes: normal fasting glucose does not describe the efficiency of the entire system. Hormones, liver output, muscle uptake and pancreatic compensation all have to be considered together.

18. Fat metabolism

T3 stimulates fatty-acid mobilization and oxidation while also affecting lipid synthesis, transport and breakdown. It may seem paradoxical for the same hormone to stimulate opposing processes, but that is the nature of metabolic turnover: multiple fluxes accelerate, and the final balance depends on energy intake, insulin, catecholamines and tissue demand. In hyperthyroidism, body weight can fall despite increased appetite because expenditure rises, yet part of the loss may be muscle. In hypothyroidism, lower expenditure and salt-water retention can raise body weight, but not every kilogram gained represents new adipose tissue. Scale weight and fat mass are not interchangeable outcomes.

19. Cholesterol and the LDL receptor

One of the most clinically important links between thyroid status and metabolism is the lipid profile. T3 increases hepatic LDL-receptor expression and influences cholesterol conversion and disposal. Overt hypothyroidism can therefore raise LDL cholesterol substantially, and correcting genuine thyroid-hormone deficiency can improve the lipid profile. This does not turn thyroid hormone into a fat-loss agent or cholesterol drug for euthyroid people. The mechanism becomes therapeutically relevant when real thyroid disease is present. For a very large athlete, lipids need to be measured directly because blood pressure, sleep apnea, diet, genetics and other exposures may carry equal or greater cardiovascular importance.

20. Protein metabolism and nitrogen balance

Thyroid hormones influence both protein synthesis and protein breakdown. At physiological levels they support normal turnover and tissue adaptation. In severe hyperthyroidism, catabolic pathways can dominate and muscle loss and weakness may occur even when appetite is high. In hypothyroidism, both synthesis and degradation may slow, while muscle becomes stiff, painful and less efficient. “Faster metabolism” therefore does not mean better anabolism. Performance requires a euthyroid range that permits normal turnover without pathological catabolism. The athlete’s goal is not maximum molecular speed but coordinated remodeling that preserves muscle, tendon, nerve and cardiovascular function.

21. Skeletal muscle is a major thyroid target

Skeletal muscle expresses thyroid receptors, transporters and deiodinases, allowing local regulation of T3 signaling. Thyroid hormones influence fiber phenotype, oxidative metabolism, calcium handling and contraction speed. Hypothyroidism can cause fatigue, cramps, pain, elevated creatine kinase and lower performance. Hyperthyroidism can cause proximal weakness and loss of muscle mass. These opposite extremes make the central point clear: muscle does not need “as much T3 as possible.” It needs the right amount of thyroid signaling. Maximum strength is built on homeostasis, where metabolic machinery supports training adaptation without either metabolic suppression or hormone-driven catabolism.

22. Muscle fibers, contraction speed and energetic cost

Thyroid signaling influences myosin isoform expression and contractile properties. Excess hormone can shift tissues toward faster behavior, but at the price of greater energy turnover and catabolism; deficiency can slow contraction and relaxation. These effects are physiologically real but do not imply a predictable performance advantage from hormone manipulation. Strongman requires strength, power, stability, recovery and tolerance of heavy training, not contraction speed in isolation. Artificially changing one parameter can degrade other links, especially cardiovascular stability, thermoregulation and muscle mass. A faster molecular clock is not useful if the organism carrying it becomes weaker or less resilient.

23. The heart: an organ that feels thyroid status immediately

Thyroid hormones increase heart rate, contractility and adrenergic responsiveness, while excess hormone lowers systemic vascular resistance and can markedly increase cardiac output. Hypothyroidism may produce slower heart rate, reduced contractility and a less favorable lipid profile. These effects are critical in strength sports because maximal lifts already generate very large blood-pressure spikes. Hyperthyroidism is not an isolated “metabolic boost”; it adds workload to a cardiovascular system that is operating close to its limits during heavy events. The calories burned are a trivial consideration compared with rhythm stability, myocardial demand and the ability to recover safely from extreme pressure loads.

24. Oxygen demand, ventilation and exercise tolerance

Greater tissue metabolism raises oxygen demand and carbon-dioxide production. In hyperthyroidism, ventilation and cardiac output must support this demand, while muscle weakness can still limit performance. In hypothyroidism, cardiac and muscular responses to exercise may be slower, and tolerance can fall through combined cardiovascular, neuromuscular and cellular mechanisms. Exercise research emphasizes this multifactorial picture. Thyroid function does not determine VO2 or maximal strength by itself, but it can shift the entire physiological system toward better or worse operating conditions. Normal thyroid status is enabling infrastructure rather than an ergogenic shortcut.

25. Body weight: thyroid matters, but it does not explain everything

Hypothyroidism is often associated with modest weight gain, with part of that change coming from salt and water retention rather than fat alone. Hyperthyroidism may cause weight loss through greater energy expenditure, but the loss can include valuable muscle tissue. In the general population, large weight changes are rarely explained by thyroid function alone. In Strongman, fluctuations may simultaneously reflect glycogen, sodium, hydration, muscle mass, adipose tissue and inflammation. Using TSH as a universal explanation for the scale oversimplifies a far more complex energy balance. Body weight is an output of multiple compartments, not a thyroid assay in kilograms.

26. Overt hypothyroidism

Primary overt hypothyroidism is typically characterized by elevated TSH and low free T4. Symptoms can include fatigue, cold intolerance, constipation, dry skin, bradycardia, weight gain, weakness and cognitive slowing, although presentations vary widely. Metabolically, energy expenditure falls, lipid turnover changes and LDL can rise. For athletes, the key problem is symptom nonspecificity. Fatigue and poor performance can also result from inadequate sleep, caloric restriction, overtraining, anemia or illness. Diagnosis therefore rests on laboratory testing and medical assessment, not on the subjective impression of having a “slow metabolism.” A symptom is a clue, not a thyroid diagnosis.

27. Overt hyperthyroidism

In hyperthyroidism, TSH is usually suppressed while free T4 and/or T3 are elevated. Palpitations, tremor, anxiety, sweating, heat intolerance, weight loss, faster bowel transit and muscle weakness may occur. Basal metabolism and substrate turnover rise, but this acceleration is not the same as improved performance. Catabolism, arrhythmias, dehydration and loss of strength can turn a “fast metabolism” into a serious physiological problem. In a strength sport where muscle mass and cardiovascular stability are essential, hyperthyroidism is more accurately viewed as impaired homeostasis running at high revolutions rather than as a desirable metabolic state.

28. Subclinical hypothyroidism

Subclinical hypothyroidism generally means elevated TSH with free T4 still within the reference interval. Not every case behaves the same way and not every case automatically requires treatment. Age, degree of TSH elevation, symptoms, thyroid antibodies, pregnancy and cardiovascular risk can alter clinical decisions. In sport, there is a temptation to turn any non-ideal TSH value into an explanation for fatigue or a plateau. Evidence that treating mild subclinical disease meaningfully enhances athletic performance is far weaker than claims made in optimization culture. Medicine treats a clinical condition and its risks, not a laboratory value merely because it looks aesthetically imperfect.

29. TSH: the best starting test, not an absolute verdict

For most people with an intact pituitary, TSH is the most sensitive initial test for primary thyroid dysfunction. A relatively small change in free T4 can generate a larger change in TSH, making it an early-warning signal. There are important exceptions: pituitary disease, pregnancy, medications, acute illness and laboratory interference can alter interpretation. TSH should therefore not be read as a “metabolic score.” It is one component of a diagnostic algorithm and becomes meaningful alongside free T4, history and symptoms. The value of a test comes from the question it answers, not from the number’s emotional appeal.

30. Free T4 and T3: when they add information

Free T4 represents the unbound fraction of the main hormone secreted by the thyroid and, together with TSH, helps classify most thyroid disorders. T3 measurement is especially useful when evaluating hyperthyroidism, including situations in which TSH is low and T4 is not markedly elevated. In hypothyroidism, T3 may remain normal until relatively late and is generally not the preferred screening test. This conflicts with online cultures that treat “high T3” as synonymous with a high-performance metabolism. Clinical endocrinology evaluates the relationship between pituitary feedback and circulating hormone, not a competition to maximize one analyte.

31. Reverse T3: a real metabolite that is easily overinterpreted

Reverse T3 is produced when T4 is inactivated and can rise during severe illness, energy restriction and other states in which thyroid-hormone metabolism shifts. It is biologically real, but its routine clinical usefulness is limited in most standard thyroid evaluations. Online discussions sometimes turn rT3 into a universal explanation for fatigue, weight-loss plateaus or a supposedly “blocked metabolism.” That interpretation goes beyond the evidence. Deiodinases adapt thyroid signaling to physiological context, and a change in one metabolite does not automatically prove a disease that requires pharmacological treatment. Adaptation and pathology are not interchangeable concepts.

32. Thyroid antibodies and autoimmunity

In Hashimoto thyroiditis, thyroid peroxidase and thyroglobulin antibodies can support the diagnosis of autoimmune thyroid disease. In Graves disease, TSH-receptor antibodies are diagnostically important. Antibodies do not directly measure metabolic rate and do not by themselves show how much thyroid hormone reaches tissues. A person may have positive antibodies while thyroid function is still normal, whereas another may have overt dysfunction. For athletes, the distinction matters because autoimmunity calls for endocrine context, not an attempt to “repair metabolism” with supplements or hormones based on one antibody result. Immune status and hormone output are related but separate questions.

33. Iodine: essential, but more is not better

Iodine is indispensable for T4 and T3 synthesis. Severe deficiency can limit hormone production and lead to goiter or hypothyroidism. Excess iodine, however, can disturb thyroid function in susceptible individuals and precipitate either hypo- or hyperthyroidism. This is a classic endocrine example in which “more” is not synonymous with “healthier.” A varied diet and ordinary iodine sources are sufficient for most people. Aggressive supplementation without indication does not transform a normal thyroid into a more powerful metabolic engine. A necessary substrate has an optimal range; beyond that range it can become a stressor rather than a benefit.

34. Selenium and the deiodinases

Deiodinases are selenoproteins, a fact that has helped create an entire supplement market claiming to “improve T4-to-T3 conversion.” Selenium is indeed required for normal function of these enzymes and other antioxidant systems, but physiology does not follow the equation “more selenium equals more T3.” Deficiency should be corrected, whereas excess intake can be toxic. Peripheral conversion is also regulated by energy availability, illness, temperature, hormones and tissue-specific expression of D1, D2 and D3. A micronutrient that is necessary for an enzyme is not automatically a metabolic accelerator when intake already meets requirements.

35. Caloric restriction and falling T3

During prolonged energy restriction, the body can reduce T3 and decrease activity of the hypothalamic-pituitary-thyroid axis as part of energy conservation. Leptin falls, central signals change and peripheral conversion can adapt. This response is not necessarily thyroid disease. It can be a physiological adjustment to reduced fuel availability. An athlete dieting aggressively may therefore experience cold intolerance, fatigue and lower performance without having primary hypothyroidism. Treating a normal adaptation as a defective gland can push the system in the opposite direction and create genuine hormone excess. Energy conservation is an evolved response, not proof that the thyroid has “shut down.”

36. Adaptive thermogenesis during weight loss

As body weight and calorie intake fall, energy expenditure may decline by more than would be predicted from the loss of tissue alone. This is adaptive thermogenesis and involves sympathetic activity, leptin, the thyroid axis and tissue-level changes. Recent research even discusses skeletal-muscle deiodinases as part of energy-conservation mechanisms during weight loss and regain. The important point is that this does not mean metabolism has been permanently “damaged.” It is a biological response to energy deficit. Refeeding and weight stabilization can alter the system again. Thyroid hormones are not a safe shortcut for overriding a normal conservation response.

37. Obesity can alter TSH without true primary hypothyroidism

People with obesity may have mildly elevated TSH while free thyroid hormones remain normal. Modern evidence suggests that some of these changes are adaptive and related to leptin, inflammation and altered deiodinase activity rather than primary failure of the gland. TSH can fall after weight loss. This is particularly relevant in Strongman, where very high body mass should not automatically be equated with thyroid disease. Diagnosis must be separated from adaptation to unusual physiology. Body mass can influence the thyroid axis, and thyroid signaling can influence metabolism, creating a bidirectional loop rather than a simple one-way cause.

38. Leptin links adipose tissue to the thyroid axis

Leptin is secreted mainly by adipose tissue and informs the brain about energy stores. During caloric restriction and fat loss, falling leptin contributes to energy-conservation responses, including reduced drive through the thyroid axis. In obesity, leptin is often high while signaling can become resistant. The thyroid axis therefore does not operate independently of body composition. The brain integrates stored energy, food intake, temperature and stress before adjusting expenditure. This helps explain why identical diets can generate different endocrine responses in different people. The system is adaptive because it integrates context, not because every individual has a fixed “metabolic speed.”

39. Circadian rhythm, sleep and TSH

TSH follows a circadian rhythm and varies across the day and night. Sleep deprivation, night shifts, stress and illness can modify the profile and add noise to laboratory testing. For an athlete, blood drawn after a sleepless night, long travel or a brutal contest is not equivalent to blood drawn under stable conditions. This is why trends and standardized collection conditions are often more useful than obsession over a single value. Thyroid function participates in the body’s rhythms, and the laboratory captures only a snapshot of a moving system. Repeatability matters when trying to distinguish biology from noise.

40. Cold exposure and local thyroid activation

Cold exposure activates sympathetic pathways and thermogenic mechanisms, including brown adipose tissue. D2 can increase local conversion of T4 to T3 and amplify thermogenic signaling without requiring a spectacular rise in circulating T3. This illustrates the sophistication of tissue regulation: the body can intensify thyroid action precisely where it is needed. Cold exposure, however, is not a method for “training the thyroid” and does not treat endocrine dysfunction. It is a physiological challenge to which the thyroid axis and nervous system respond together. Local hormone activation is adaptation, not evidence that more systemic hormone would be better.

41. What a hard training session can do to thyroid tests

Intense exercise can temporarily change TSH, T3 and T4, with the direction depending on duration, intensity, energy status, temperature and timing of the blood draw. During prolonged work or energy deficit, T3 may fall as part of adaptation. A maximal Strongman session also produces inflammation, sympathetic stress, plasma-volume shifts and large glycogen use, all of which can influence laboratory context. Endocrine tests intended to investigate a chronic issue are therefore easier to interpret when collected during a relatively stable period rather than immediately after an exhausting competition. Timing is part of test quality.

42. Strength and thyroid function: the relationship is not linear

There is no evidence that progressively higher T3 within or beyond the physiological range automatically produces greater strength. Performance depends on contractile mass, motor-unit recruitment, technique, leverage, glycogen and tissue health. Both hypothyroidism and hyperthyroidism can reduce strength through different mechanisms. With deficiency, contraction and energy production slow; with excess, catabolism, weakness and cardiovascular stress can dominate. The optimal curve is closer to a broad euthyroid plateau than to a line in which every extra unit of hormone adds kilograms to the bar. More hormonal signal is not synonymous with more useful force.

43. Endurance and aerobic capacity

Thyroid hormones affect cardiac output, mitochondrial function, ventilation and substrate use, so dysfunction can reduce exercise tolerance. In untreated hypothyroidism, cardiovascular, muscular and metabolic limitations can overlap. In hyperthyroidism, oxygen consumption may already be high at rest while exercise efficiency and cardiovascular reserve are impaired. For Strongman, even though many events are short, aerobic capacity still matters for recovery between efforts, oxygen delivery and clearance of metabolites. A normal thyroid supports this infrastructure but does not substitute for conditioning. The endocrine system creates the operating environment; training determines much of what the athlete can do inside it.

44. Acute illness and non-thyroidal illness syndrome

Severe illness, trauma, infection or systemic stress can profoundly change thyroid-hormone metabolism even when the thyroid gland is not the primary problem. T3 may fall, reverse T3 may rise and, in severe cases, TSH and T4 can also change. This pattern is called non-thyroidal illness syndrome or euthyroid sick syndrome. Its interpretation is difficult and it should not automatically be labeled primary hypothyroidism. A serious infection, major injury or postoperative period in an athlete can therefore produce a transient profile. Clinical context matters more than an attempt to force every number back into a reference interval.

45. Strongman: very high body mass changes the metabolic context

In the Open class, physiology operates at an unusual scale: enormous muscle mass, high calorie intake, variable visceral fat, substantial heat production and high cardiovascular load. TSH and energy needs must be interpreted in that setting rather than through simplistic comparisons with a sedentary 70-kilogram adult. Lean mass increases energy expenditure, while adipose tissue and leptin can influence the thyroid axis. The thyroid is important, but it cannot alone explain why a large athlete may require four, five or more thousand calories in a given training phase. Size, activity, digestion, thermogenesis and tissue maintenance all contribute.

46. Glycogen and recovery between sessions

Thyroid hormones affect carbohydrate turnover, but glycogen restoration after training depends primarily on carbohydrate intake, insulin, blood flow, prior contraction and recovery time. Abnormal thyroid status can modify this metabolic background, but T3 is not a “glycogen transporter.” In Strongman, where one session may combine deadlift, yoke, farmers and log, substrate restoration is essential. If performance declines, total energy, carbohydrates, sleep and workload should be evaluated before the thyroid becomes the automatic suspect. Hormonal context matters, but practical recovery physiology still begins with fuel, time and adequate training organization.

47. Heat, sweating and hydration

Hyperthyroidism increases heat production and can amplify sweating, heart rate and the need for heat dissipation. In a very large athlete, the ratio between body mass and available cooling surface is already unfavorable, while equipment, hot environments and intense effort make the problem worse. Dehydration adds further cardiovascular stress. Thyroid-hormone excess can therefore create a dangerous combination even when it is initially perceived as “energy” or a “good metabolism.” Thermoregulation is a physiological requirement, not a cosmetic bonus of burning calories. The athlete must survive the heat produced by the metabolic machinery before any performance benefit can even be considered.

48. Why thyroid hormones are not weight-loss drugs for euthyroid people

Giving thyroid hormone to someone with normal thyroid function can create iatrogenic thyrotoxicosis. Body weight may fall, but the loss can include muscle, while risks include tachycardia, arrhythmias, anxiety, tremor, heat intolerance, bone effects and cardiovascular strain. The body also responds by suppressing TSH and endogenous thyroid output. Using T3 or T4 as a cutting tool means buying extra energy expenditure at the price of endocrine imbalance. This article explains the mechanism but does not provide doses, cycles or self-medication protocols. Treatment of genuine thyroid disease and pharmacological weight manipulation are fundamentally different goals.

49. T3 is not simply a “better” version of T4

Because T3 is more active at the receptor, it is tempting to consider it superior. In normal physiology, however, T4 provides a relatively stable reservoir while tissues regulate local conversion through deiodinases. Direct T3 exposure has a different pharmacological profile, with faster changes and greater potential for tissue excess. In medical treatment of hypothyroidism, levothyroxine remains standard therapy for most patients, while approaches that include T3 are reserved for selected situations and remain debated. “More active” does not mean “more physiological,” and it certainly does not mean “better for performance.” The delivery pattern is part of the biology.

50. Cardiovascular risk of thyroid-hormone excess

Excess thyroid hormone can increase heart rate, pulse pressure and myocardial workload and is associated with arrhythmia risk, including atrial fibrillation in susceptible populations. In strength sport, that risk overlaps with extreme blood-pressure spikes, adrenergic activation, dehydration and sometimes very high body mass. The combined load matters far more than the extra calories burned. Persistent palpitations, chest pain, unexplained shortness of breath or unusual exercise intolerance deserve medical assessment rather than empirical hormone adjustment. A cardiovascular system under endocrine stress is not a performance-enhanced system; it is a system with less margin for error.

51. Bone and thyroid function

Bone turnover accelerates in hyperthyroidism, and prolonged hormone excess can reduce bone mineral density and increase fracture risk. In hypothyroidism, remodeling slows, but that does not automatically produce “stronger” bone. Skeletal quality depends on a balance between formation and resorption, mechanical loading, vitamin D, calcium and other hormonal systems. For Strongman, bone is the foundation through which enormous forces are transmitted. Weakening this infrastructure for a temporary metabolic effect is biologically inconsistent with the sport’s long-term objective. Performance requires tissues that can tolerate repeated loading, not only a lower number on the scale.

52. “Thyroid support” supplements and hidden complexity

Products marketed for “thyroid support” may contain iodine, selenium, glandular extracts, stimulatory herbs or poorly standardized mixtures. Historically, some products have been reported to contain detectable thyroid hormones or ingredients capable of interfering with thyroid function. A supplement label does not guarantee metabolic neutrality. Tested athletes also face contamination risk from unrelated prohibited substances. A normal thyroid does not need an obscure cocktail to become “more optimal.” When thyroid disease is suspected, the appropriate tools are laboratory testing, clinical assessment and an endocrinologist rather than an ingredient list designed around marketing language.

53. Laboratory interference: biotin and other traps

Some thyroid immunoassays can be affected by high-dose biotin, producing patterns that mimic hyperthyroidism, such as apparently low TSH and apparently elevated thyroid hormones depending on the assay platform. Other interfering antibodies and analytical factors can also distort results. A surprising test should therefore be correlated with symptoms and may need repeating or confirmation by another method. In sports nutrition, biotin may appear in hair, skin or multivitamin products, so a complete supplement history matters. Laboratory medicine is part of endocrinology: a number is only as trustworthy as the method that produced it.

54. Trends are more useful than hunting for a perfect number

TSH and free T4 become more informative when interpreted over time under comparable conditions. A single result can be influenced by illness, sleep, time of day, laboratory method, medications and changes in energy availability. Athletes benefit from avoiding routine monitoring immediately after extreme competitions, sleepless nights or acute illness whenever the purpose is to understand a chronic state. A trend can show whether a change is persistent or simply biological noise. The goal is not to obtain an aesthetically pleasing TSH but to identify stable thyroid function compatible with health, recovery and performance.

55. Myth: “my metabolism is slow” always means thyroid disease

The phrase “I have a slow metabolism” is used for fatigue, weight-loss plateaus, cold sensations or difficulty losing fat. Sometimes genuine hypothyroidism is present, but often the explanation is broader: underestimated calorie intake, reduced spontaneous movement, diet adaptation, poor sleep, stress, medications or body composition. Thyroid function should be tested when appropriate, but turning the gland into the villain behind every plateau is not useful. Energy metabolism is the sum of hundreds of processes. TSH can help diagnose a specific endocrine problem; it cannot explain the entire lived experience of body weight by itself.

56. Myth: high T3 means perfect metabolism

An elevated T3 concentration can occur in hyperthyroidism and can increase energy expenditure, but that is not equivalent to health or performance. A system running too fast consumes more fuel while also generating more heat and stress. Biologically, excessive T3 can cause tachycardia, muscle catabolism, tremor, insomnia and bone loss. In strength sport, those effects damage exactly the infrastructure required for performance. The endocrine goal is euthyroidism: enough signal for normal tissue function, without deficiency and without excess exposure. Maximum hormone concentration is not the same target as maximum useful human performance.

57. Myth: a normal TSH excludes every possible problem

A normal TSH makes primary hypothyroidism unlikely in most people with an intact pituitary, but it is not an absolute rule for every clinical situation. Central hypothyroidism, selected medications, severe illness and analytical interference can disrupt the usual relationship between TSH and free T4. Symptoms may also come from completely different systems even when thyroid function is normal. A good test is not a universal explanation. It answers a precise question. Correct medicine begins when the clinical question and the test are matched rather than when every symptom is forced into a thyroid narrative.

58. Which tests make sense in a basic evaluation

For ordinary suspicion of primary thyroid dysfunction, TSH is usually the starting point, with free T4 added when TSH is abnormal or the clinical context requires it. T3 can be useful in hyperthyroidism. Thyroid antibodies are used when autoimmunity is suspected, while ultrasound answers structural questions rather than measuring metabolic speed. The exact set of tests depends on the clinician and the problem. Ordering TSH, FT4, FT3, reverse T3, multiple antibodies and micronutrients without a clear question can create more confusion than information. Testing should be hypothesis-driven, not a laboratory shopping list.

59. How the thyroid connects with insulin, estrogen and IGF-1

This series began with insulin and moved through hormonal modulators because endocrine axes do not operate independently. T3 influences glucose and lipid flux; insulin regulates nutrient availability and anabolic signaling; estrogens affect insulin sensitivity and body composition; GH and IGF-1 interact with substrate use and growth. The liver and skeletal muscle receive all of these messages simultaneously. When one hormone changes, the final effect depends on the others. Endocrinology is a network of feedback loops, not a collection of independent knobs. The body integrates signals, and performance emerges from the combined state of the system.

60. Conclusion: the thyroid sets the tempo, but it does not write the whole score

The thyroid is one of the great regulators of energy metabolism, but it is not the only one. TSH, T4, T3, transporters, deiodinases and receptors form a network that adapts energy expenditure to temperature, nutrition, illness and activity. For athletes, the central message is simple: normal thyroid function supports the heart, muscle, thermoregulation and fuel use, while deficiency and excess reduce performance through different pathways. Hormonal manipulation for weight loss or a supposedly faster metabolism cannot replace nutrition, sleep, conditioning and medical evaluation. The system performs best when the tempo is appropriate, not when every metabolic dial is turned to maximum.

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Editorial note: this material is educational and does not constitute medical advice, diagnosis or an administration protocol for levothyroxine, liothyronine or other thyroid hormones. Inappropriate pharmacological manipulation of the thyroid axis can affect the heart, bone, muscle, thermoregulation and other systems.