Thyroid

The thyroid is a gland in the front of the neck that releases thyroxine (T4) and triiodothyronine (T3). Those hormones set oxidative metabolism in tissues: ATP production, CO₂ retention, warmth, and resistance to stress chemistry. A shortage of thyroid hormone at the cell level lies behind much chronic disease. The healthy gland secretes about three parts T4 to one part T3, and the liver converts more T4 to active T3 when energy is abundant. Standard blood tests can miss a shortage inside cells: TSH can look normal while tissues are underserved, because the pituitary uses different hormone transporters than the rest of the body. Large cohort data put the lowest cardiovascular and mortality risk at mid-range TSH, with the lowest fifth carrying higher all-cause mortality.

Broda Barnes put thyroid at the center of chronic disease. People who escape tuberculosis while hypothyroid still die later of cancer or heart disease in his clinical picture. Otto Warburg's respiratory defect in cancer lines up with thyroidectomy animals developing atherosclerosis and with high serum cholesterol in hypothyroidism.

Inflammation, aging, and degeneration involve impaired oxidation. Restoring respiration counters lipid peroxidation chains. T3 specifically is the top-level hormonal regulator of mitochondrial quality control (biogenesis and mitophagy); its age-related decline drives mutant mitochondrial DNA accumulation. See Triiodothyronine.

T4, T3, and the liver

Thyroid hormone is essential for maintaining adequate carbon dioxide production, for minimizing lactic acid, cortisol and prolactin, for regulating calcium and magnesium, for avoiding hypotonicity of the body fluids, and for improving the ratio of palmitic acid to linoleic acid.

— Ray Peat

raising your body temperature is one of the basic essential things that the thyroid hormone does

— Ray Peat

Glucose supports the conversion of T4 to T3, and fasting drops metabolic rate sharply.

In fasting, the liver's glycogen at first keeps T3 production going; once it runs out, mobilized fatty acids interfere with glucose use and certain amino acids suppress the gland, and eating carbohydrate, especially fruit, lets the liver resume production. The Biskinds showed in the 1940s that a liver short of thyroid, protein or vitamins lets estrogen rise sharply.

Estrogen inhibits thyroid secretion. Progesterone promotes release from the gland. Women with high estrogen load and low progesterone show more hypothyroid signs even with "normal" labs.

Cholesterol

By the mid-1930s, it was generally known that hypothyroidism causes the cholesterol level in the blood to increase; hypercholesterolemia was a diagnostic sign of hypothyroidism. Administering a thyroid supplement, blood cholesterol came down to normal exactly as the basal metabolic rate came up to the normal rate.

— Ray Peat

With enough vitamin A and thyroid hormone, cholesterol is converted massively into pregnenolone and the other protective steroids; a deficiency of either lets blood cholesterol rise, a sign of too little DHEA and testosterone.

Basal temperature and pulse still beat TSH alone for many practical decisions. Healthy populations average a resting pulse of about 85 per minute. See Basal temperature, Cholesterol, Hypothyroidism.

Stress and reverse T3

Stress, thyroid deficiency and low protein intake typically raise estrogen, and stored PUFA adds to its action.

Low sugar, high free fatty acids, endotoxin, and PUFA push T4 toward inactive reverse T3 and away from T3. The result is low tissue thyroid with confusing serum numbers. See Randle cycle, Endotoxin, PUFA, Stress.

In zinc-deficient patients with low free T3, zinc repletion normalized free T3 and T3, lowered reverse T3, and normalized the pituitary TSH response after a releasing-hormone challenge without changing selenium. See Zinc, Reverse T3.

Salt loss rises when thyroid is low. Adrenaline climbs. Salt and glucose support the recovery set. See Salt, Glucose.

In 12 thyroidectomized patients tracked through six weeks of L-T4 withdrawal and two months of resumed treatment, induced hypothyroidism raised noradrenaline by about 88% (2954 vs. 1574 pmol/L), adrenaline by about 105% (228 vs. 111 pmol/L), aldosterone by about 98% (310 vs. 157 pmol/L), and cortisol by about 63% (409 vs. 251 pmol/L), alongside a rise in daytime diastolic blood pressure that reversed once thyroid hormone treatment resumed.

Cellular hypothyroidism

In 10 obese adults after 7 days on a 240 kcal diet, serum total T4 held steady while free T4 rose, yet mass transfer of T4 into rapidly and slowly equilibrating tissue pools fell and tissue pools shrank; T3 production and tissue transfer also fell, with transport of T4 into tissues reduced more than T3. Serum free-hormone changes alone do not account for the smaller tissue pools, so diminished membrane transport contributes to the low-T3 pattern of caloric restriction. Thyroid hormone transport across the cell membrane, beyond glandular output or serum levels alone, determines intracellular T3. Reduced T4/T3 transport into peripheral tissue is associated with insulin resistance, diabetes, depression, bipolar disorder, chronic fatigue, fibromyalgia, neurodegenerative disease, migraines, stress, anxiety, chronic dieting, and aging, while intracellular T3 in the pituitary, which has different transporters than the rest of the body, often stays unaffected. Because the pituitary keeps sensing adequate T3 even when peripheral tissue does not, TSH can look normal despite widespread tissue-level hypothyroidism, making TSH and standard serum panels a poor marker for whether cells are actually getting enough thyroid hormone. This can produce patients who test "euthyroid" but do not tolerate or respond to standard-dose thyroid hormone supplementation, since the ceiling on benefit is transport into cells rather than hormone availability in the blood. A large individual-participant-data meta-analysis of 134,346 people found the lowest risk of cardiovascular disease and mortality at TSH 1.90-2.90 mIU/L (60th-80th percentile), while TSH in the lowest 20th percentile carried higher all-cause mortality risk, evidence against treating suppressed TSH as a safe target. A NHANES cohort study of over-50 adults similarly found TSH unrelated to mortality while free T3 was protective and free T4 harmful, pointing at T3 specifically as the hormone that tracks longevity. See Triiodothyronine.

Support and cautions

Support: adequate calories and carbohydrate, protein with gelatin balance, liver and eggs for vitamin A and minerals, light and sleep, progesterone when estrogen-dominant, careful desiccated thyroid or T3/T4 under monitoring.

Cautions: starting dose too high; adrenal and low-calorie contexts; iodine excess or deficiency extremes; selenium and iron status. Troubleshoot bad reactions by checking pulse, temperature, rest, food, and estrogen load. See Harm reduction.

Sampling time and TSH

In a retrospective analysis of 80,150 outpatient TSH measurements, sampling time had the largest effect among sex, age, season, and clock time: TSH fell from 7 a.m. to about 1 p.m., then rose again through 4 p.m. A morning value near the low end of the reference range can look "normal," while the same person sampled at midday can fall below the cut-point and be labeled abnormal.

Related findings

Anti-thyroid drug hypothyroidism involutes the rat thymus over weeks (cell death, thymocyte loss, tissue remodeling).

In predialysis chronic kidney disease with metabolic acidosis (serum total CO₂ ≤ 22 mM), oral sodium bicarbonate titrated to raise total CO₂ to ≥ 24 mM over 8–12 weeks raised free T3, total T3, free T4, and TSH, while free T3 fell further in untreated controls; percentage rise in total CO₂ tracked the T3 changes, and GFR held in the bicarbonate arm but fell in controls. Baking soda (sodium bicarbonate) was the intervention. See Baking soda, CO2.

Experimental hypothyroidism raised skin and lacrimal-gland mast cell counts up to 4.5-fold and histamine content up to 50%, with more degranulation.

Cold exposure intensifies thyroid histology (capillary congestion, follicular colloid loss) and can push toward hyperthyroidism; heat stress slows the gland and leaves colloid accumulated in the alveoli.

Near-infrared photobiomodulation plus vitamin D/selenium improved TSH and thyroid antibodies and reduced levothyroxine need in Hashimoto thyroiditis versus supplements alone; see Red light, Hashimoto.

Broda Barnes framed thyroid deficiency as the most potent factor in the development of atherosclerosis and heart attacks in Hypothyroidism: The Unsuspected Illness (with Lawrence Galton, 1976).

Perchlorate in drinking water inhibits iodide uptake by the thyroid at low concentrations. Several UV-filter benzophenones used in sunscreens and other personal-care products, including benzophenone-1, down-regulated thyroid receptor beta and other thyroid-signaling genes (Tshβ, Trhr) in rat pituitary (GH3) cells and disrupted thyroid follicular gene expression in a separate thyroid cell line, with confirmatory effects in zebrafish embryos for the most potent compounds tested.

In a cross-sectional survey of 600 people using a machine-learning (SHAP/XGBoost) analysis, longer daily Bluetooth headset usage duration was one of the two strongest factors associated with thyroid nodule presence, alongside age; the study design (self-reported questionnaire, no causal mechanism established) means this is an association worth further study rather than a proven risk. Maternal thyroid status shapes offspring development. Thyroid hormone modulates the brain serotonin system: in hypothyroid states serotonergic tone is blunted, while restoring T3 desensitizes inhibitory 5-HT1A autoreceptors in the raphe and increases 5-HT2 receptor sensitivity, disinhibiting cortical and hippocampal serotonin release, which is one proposed mechanism for T3's mood and exploratory-behavior effects and its use as an antidepressant augmentation agent. Diet composition (PUFA, energy) changes T4-to-T3 conversion. Natural desiccated thyroid versus levothyroxine remains a practical clinical debate.

In a doubly-labeled-water study comparing 150 healthy underweight Chinese adults (BMI below 18.5) to 173 normal-BMI controls, the underweight group had lower food intake and lower physical activity, not higher activity as researchers expected, yet showed higher-than-predicted resting energy expenditure tied to elevated thyroid hormone levels.

Thyroid and progesterone can restore the capacity to feel pleasure and love after long deficiency:

A woman said, soon after she began using progesterone and thyroid, that, for the first time in her life she felt that her heart and abdomen were opening with feelings of love. Another said "thyroid is love." Since these women were using the hormones for problems that had developed gradually, over several years if not their entire life, the sudden change was dramatic. They had forgotten that life itself was pleasurable.

— Ray Peat

Hyperthyroidism

Triiodothyronine (T3) molecular structure

Hyperthyroidism is excess thyroid hormone effect: high pulse, heat, weight loss, anxiety. Graves disease and excess dosing are common causes. True hyper differs from compensatory high pulse in hypothyroidism with adrenaline. Blood viscosity is a marker of the thyroid-progesterone-estrogen axis: hyperthyroid people have unusually fluid blood, while hypothyroid people (low progesterone, high estrogen) have abnormally viscous blood and greater susceptibility to heart disease and cancer.

In rats, thyroid extract, thyroxine, or triiodothyronine lowered voluntary alcohol intake.

Graves' disease

Graves' disease is an autoimmune condition in which antibodies stimulate the thyroid-stimulating hormone receptor directly, causing hyperthyroidism independent of normal pituitary control; it is the most common cause of hyperthyroidism. In Peat's general framework, true hyperthyroid states like Graves differ from the compensatory high-adrenaline states mimicking some hyperthyroid symptoms in genuinely hypothyroid people. The two can look similar symptomatically (elevated pulse, anxiety, heat intolerance) but require opposite treatment approaches, which is why self-diagnosis from symptoms alone is unreliable here: Graves' disease requires medical diagnosis (antibody testing, not just symptoms or a single temperature/pulse reading) and medical management (antithyroid drugs, radioactive iodine, or surgery), not something to self-manage with diet alone. Third-generation TSH-receptor-antibody (TRAb) assays have over 95% sensitivity and specificity for diagnosing Graves' disease; a TRAb level above 12 IU/l at diagnosis predicts a 60% relapse risk at 2 years and 84% at 4 years after stopping antithyroid drugs.

Practice (section 1)

Medical diagnosis required. Do not self-treat with random thyroid doses. See Thyroid, Hypothyroidism, Adrenaline, Pulse, Harm reduction.

Hypothalamus–pituitary

The hypothalamus links the brain to the endocrine system, controlling temperature, appetite, thirst, and the pituitary gland below it. It integrates whole-body energy status into hormonal output.

Fat type shapes hypothalamic signaling

Saturated fat and PUFA have opposing effects on the hormone systems the hypothalamus governs, pituitary hormone regulation among them:

The different fatty acids are directly involved, very often with opposite effects, in cell division and growth, cell stability and dissolution, the organization of cells, tissues, and organs, the regulation of pituitary hormones, adrenalin and sympathetic nervous activation, histamine and serotonin synthesis, adrenal cortex hormones, thyroid hormones, testosterone, estrogen, activators of the immune system and inflammation (cytokines), autoimmune diseases...

— Ray Peat

Extreme outputs

When systemic energy is low or inflammation is high, the hypothalamus produces protective but disruptive outputs: fever, appetite loss, and reproductive shutdown (functional hypothalamic amenorrhea in women, driven by energy availability more than any single hormone). Energy availability below roughly 30 kcal/kg fat-free mass per day suppresses LH pulsation, and body fat above 22% is often needed to restore menstruation; each added kilogram of body fat raises the odds of resuming periods by about 8%. These are downstream signs that the hypothalamus is responding to a real systemic energy deficit, not a primary hypothalamic disease in most cases.

Practice (section 2)

Supporting systemic energy (adequate food, carbohydrate, low PUFA) supports hypothalamic function indirectly, rather than targeting the hypothalamus itself. See Pituitary, Thyroid.

Reverse T3

Reverse T3 (rT3) is an inactive product of thyroxine (T4). Under stress, low glucose, high cortisol, and high free fatty acids, more T4 becomes reverse T3 instead of active triiodothyronine (T3). Tissue thyroid effect falls while labs can still look "normal."

History (section 1)

Etymology (section 1)

The name describes its relationship to T3: it is a structural mirror, an isomer with the same three iodine atoms attached at a different position on the thyronine ring, produced instead of active T3 under certain conditions.

Reverse T3 was identified in the 1970s once chromatography methods became sensitive enough to distinguish it from active T3 in blood.

Reverse T3 was established as a distinct, naturally-occurring T4 metabolite in the early 1970s, once chromatographic methods sensitive enough to separate it from T3 became available. It gets its name from having the same three iodine atoms as T3 but attached at the mirror-image (3,3',5') ring positions instead of T3's own (3,5,3') pattern, making it a "reversed" isomer of the active hormone rather than a wholly different molecule.

Structure/Chemical properties (section 1)

Reverse T3 shares T3's molecular formula (C₁₅H₁₁I₃NO₄) and the same three-iodine count as thyroxine's other deiodination product, but the iodine on the outer (phenolic) ring sits at the 3' position instead of 5'. That shift in iodine placement is enough to abolish activity at the thyroid hormone receptor while still letting rT3 occupy transport and enzyme binding sites meant for T3.

Function/Mechanism of Action (section 1)

T4 is deiodinated into either active T3 (removal of an iodine from the outer ring, via type I or II deiodinase) or inactive reverse T3 (removal from the inner ring, via type III deiodinase). Which path dominates is not fixed: cortisol, adrenaline, low glucose, and high free fatty acids all shift conversion toward reverse T3 and away from T3. Because reverse T3 competes with T3 for the same deiodinase enzymes and transport proteins without activating the thyroid hormone receptor, elevated rT3 can blunt T3's effects even while total or free T3 measurements look adequate.

Zinc repletion in zinc-deficient patients with low free T3 lowered reverse T3 while restoring free T3/T3 and the pituitary TSH response after a releasing-hormone challenge. See Zinc.

A 5-day military ranger training course combining heavy physical exercise with severe caloric restriction showed this shift directly: in 24 young, fit cadets, T3 fell and reverse T3 rose together as the course wore on, while TSH stayed suppressed throughout rather than rising to compensate. The same combination of eating less and exercising more that is often given as generic health advice reproduced a lab pattern indistinguishable from starvation-adapted low thyroid function.

Because the actions of T3 can be inhibited by many factors, including polyunsaturated fatty acids, reverse T3, and excess thyroxine, the absolute level of T3 can't be used by itself for diagnosis.

— Ray Peat

Clinical significance (section 1)

Reverse T3 is not administered as a drug; it has no approved clinical use and is measured only as a lab marker. A high reverse T3 relative to T3, sometimes called "T3 resistance" informally, is used by some clinicians as evidence that stress, illness, or fasting is diverting thyroid metabolism away from active T3 despite normal-looking TSH and T4. This is not a formally recognized diagnostic category in mainstream endocrinology, which generally treats reverse T3 elevation as a byproduct of illness ("low T3 syndrome" / "sick euthyroid syndrome") rather than a condition to treat directly. A 201-patient ICU cohort found 32.3% had low T3 (below 60 ng/dL) within 48 hours of admission, and those patients had substantially higher in-hospital mortality (24.6% vs. 8.1%), though the association weakened after adjusting for overall illness severity. In the bioenergetic framework, lowering the drivers, eating adequate carbohydrate, reducing polyunsaturated fat intake, and supporting T3 conversion directly, is favored over targeting reverse T3 as an isolated number.

Practice (section 3)

Lower the drivers: eat carbohydrate, cut seed oils, support thyroid with T3-containing therapy when conversion is poor, and reduce chronic stress load.

Do not chase a single rT3 number without pulse, temperature, and symptoms. See Triiodothyronine, Thyroxine, NDT, Hypothyroidism, Basal temperature.

TRH and TSH

Thyrotropin-releasing hormone is the hypothalamic signal that raises TSH. Stress and dopamine signals modulate the axis.

History (section 2)

Etymology (section 2)

TRH stands for thyrotropin-releasing hormone: the hypothalamic hormone that triggers release of thyrotropin (thyroid-stimulating hormone) from the pituitary. "Thyro-" traces to Greek thyreos ("shield-shaped," describing the thyroid gland) and "-tropin" to Greek tropos ("turning").

Roger Guillemin and Andrew Schally independently isolated and characterized TRH in 1969, the first hypothalamic releasing hormone to be identified; both shared the 1977 Nobel Prize in Physiology or Medicine for this work.

TRH was the first hypothalamic releasing hormone to be structurally identified, isolated and sequenced independently by Roger Guillemin's and Andrew Schally's laboratories in 1969. It turned out to be the simplest of the hypothalamic releasing hormones, just three amino acids (pyroglutamate-histidine-proline), work that contributed to Guillemin and Schally sharing the 1977 Nobel Prize in Physiology or Medicine.

Structure/Chemical properties (section 2)

TRH is a tripeptide, pyroglutamyl-histidyl-proline amide (pGlu-His-Pro-NH₂), with both ends of the chain chemically modified (a cyclized N-terminus and an amidated C-terminus). This modification protects it from ordinary peptide-degrading enzymes, which is part of why it survives long enough in circulation to reach the pituitary.

Function/Mechanism of Action (section 2)

TRH is released from the hypothalamus and travels through the portal blood system to the anterior pituitary, where it binds TRH receptors on thyrotrope cells and stimulates release of TSH. Dopamine has an inhibitory effect on this axis, and stress hormones and illness state more broadly suppress it, part of why acute illness lowers thyroid output through the "sick euthyroid" pattern rather than through a thyroid gland problem itself.

Cardiovascular and hormonal effects

TRH itself can cause tachycardia, palpitations, high blood pressure, stasis of the intestine, increase of pressure in the eye, and hyperventilation with alkalosis. It can increase the release of norepinephrine, but in itself it acts very much like adrenalin. TRH stimulates prolactin release, and this can interfere with progesterone synthesis, which in itself affects heart function.

— Ray Peat

Elevated prolactin from TRH can also lower testosterone by suppressing gonadotropin-releasing hormone.

Clinical significance (section 2)

The TRH stimulation test uses an injection of TRH and measures the pituitary's TSH response. It was once used to probe for subtle pituitary or thyroid dysfunction. Its practical value has been challenged: one study found it "gives many misleading results and has an elevated cost/benefit ratio as compared with the characteristic combination of low thyroxinemia and non-elevated TSH." Reported risks of the test itself have also been documented. A study of 137 thyroidectomized patients given the standard 200 mcg IV TRH dose concluded a sufficiently sensitive basal TSH assay alone could predict the TRH-stimulated response, allowing the test itself to be abandoned for routine follow-up. TRH itself (as protirelin) has been studied as an investigational treatment in epilepsy, spinal cord injury, and some neurodegenerative conditions, but has no routine approved clinical use in most of the world.

Thyroid hormones

Thyroid hormone usually means T4 (thyroxine) and T3 (triiodothyronine) from the Thyroid gland. These set metabolic rate, mitochondrial respiration, and CO₂ production.

The healthy gland releases roughly three parts T4 to one part T3 (about 80% T4, 20% T3 of glandular output). Most circulating T3 (about 80%) is made outside the thyroid by liver deiodination, and only a small fraction of either hormone circulates unbound, roughly 0.02% of T4 and 0.3% of T3. See Thyroid, Thyroxine, Triiodothyronine.

Dosing (section 1)

Protocols are on Roadmap/05 - Supplementing T3/T4 only. See NDT, Harm reduction.

History (section 3)

Etymology (section 3)

"Thyroid" comes from Greek thyreoeides ("shield-shaped"), a term coined by English anatomist Thomas Wharton in 1656 to describe the gland's shape.

The thyroid gland's role in producing hormones wasn't established until the late 19th century, when removal of the gland was linked to myxedema and cretinism, prompting the first thyroid extract treatments in the 1890s.

Thyroid extracts treated myxedema from the 1890s. Synthetic T4 dominated prescribing after the mid-20th century. T3 was identified as the major active iodothyronine in the 1950s.

Animal thyroid tissue entered Chinese goiter formulas much earlier. Sun Si-Miao's 652 AD Thousand Golden Prescriptions included seaweed plus sheep or deer "yeh" (neck and face junction tissue that likely contained thyroid gland). Western raw-thyroid treatment of post-surgical hypothyroidism was reported by Kocher in 1893; porcine tablets were common until levothyroxine after 1927.

Structure/Chemical properties (section 3)

T4 (C₁₅H₁₁I₄NO₄) carries four iodine atoms; T3 (C₁₅H₁₂I₃NO₄) has one fewer. Reverse T3 is an inactive deiodination product that rises under stress.

3,5-T2 is a smaller thyroid hormone metabolite with two iodine atoms. In rats on a high-fat diet, T2 given alongside the diet prevented insulin resistance and fat buildup in the liver, working through the enzyme SIRT1 rather than the usual thyroid receptor. In rats that already had fatty liver from six weeks of high-fat feeding, four weeks of T2 cleared much of the liver fat by speeding up fat burning in the mitochondria. In obese mice, T2 acted much like T3 instead: it suppressed the pituitary signal to the thyroid, lowered blood T4 and T3, and enlarged the heart. In a report on two healthy volunteers taking T2 for several weeks, resting metabolic rate rose and TSH, T4 and T3 stayed the same.

Function/Mechanism of Action (section 3)

T3 drives calorigenic and mitochondrial effects; T4 is a prohormone until converted.

The brain concentrates T3 from the serum, and may have a concentration 6 times higher than the serum... An excess of thyroxine, in a tissue that doesn't convert it rapidly to T3, has an antithyroid action.

— Ray Peat

In hypothyroid rats receiving a steady T4 infusion, brain-cortex T3 concentration reached six times the serum concentration and exceeded the local T4 concentration. At a lower T4 infusion, the brain-cortex-to-serum T3 ratio was 3:1, showing that brain and serum measurements can diverge substantially.

Measuring the amount of thyroid in the blood isn't a good way to evaluate adequacy of thyroid function, since the response of tissues to the hormone can be suppressed (for example, by unsaturated fats).

— Ray Peat

Estrogen inhibits thyroid secretion; Progesterone promotes hormone release from the gland.

Diet and tissue sensitivity

A high-carbohydrate, fat-free diet sharply increases tissue sensitivity to T3. In thyroidectomized rats, the T3 dose needed for half-maximal induction of liver lipogenic enzymes dropped 3-7 fold on a high-carbohydrate diet compared with controls, a true synergy rather than a simple additive effect, and independent of any change in T3 metabolism or in the number or affinity of nuclear T3 receptors.

Essential-fatty-acid-deficient and protein-deficient rats are, conversely, extraordinarily sensitive to exogenous thyroid hormone even though their own thyroid activity is not elevated.

PUFA deficiency also changes how mitochondria use the thyroid-driven respiratory increase: PUFA-deficient rat liver mitochondria run a higher respiratory rate but couple it more efficiently to ATP synthesis than PUFA-replete controls, maintaining a higher ATP/ADP-Pi ratio despite a lower membrane potential.

Stomach acid and H. pylori

In rats, thyroidectomy cut both parietal cell count and basal/histamine-stimulated stomach acid secretion roughly in half; chronic thyroxine treatment increased parietal cell mass and acid secretion above control levels, independent of autoimmune mechanisms. Low stomach acid from hypothyroidism makes the stomach more hospitable to H. pylori, which itself lowers stomach acid further and can interfere with oral thyroid hormone absorption, a bidirectional relationship comparable to the SIBO/hypothyroidism cycle discussed on SIBO.

Medical uses/Effects (section 1)

Levothyroxine (T4) is standard hypothyroidism therapy worldwide. NDT and T3 combinations remain in use when conversion is poor. Clinical signs (Basal temperature, pulse, cholesterol) are paired with labs.

Side/Adverse effects (section 1)

Overtreatment causes tachycardia and anxiety. Excess T4 without adequate T3 conversion can worsen hypothyroid symptoms. Impairing factors include PUFA, Estrogen, gut Endotoxin, and chronic Stress.

Herpes simplex

T3 suppresses HSV-1 (cold sore) replication; thyroid hormone and thyroid nuclear receptor inhibit HSV-1 key gene expression and block viral replication. Transient or chronic hypothyroidism lowers hormone level and favors viral reactivation. See Herpes simplex.

T4 (thyroxine)

Thyroxine (T4, levothyroxine) is the main hormone the thyroid gland secretes. It is a prohormone: most activity requires conversion to T3 in the liver and other tissues.

The gland releases mostly T4 and some T3; most circulating T3 is made from T4 outside the thyroid. T4 redirects here.

Dosing (section 2)

Supplement protocols, warnings, labs, and titration are on Roadmap/05 - Supplementing T3/T4. See also NDT, Triiodothyronine, Basal temperature, and Harm reduction.

History (section 4)

Etymology (section 4)

The name combines "thyroid" (see Thyroid hormone) with "oxy-" (for its oxygen-containing structure) and the amine suffix "-ine."

Edward Kendall isolated thyroxine in crystalline form in 1914, the first thyroid hormone to be identified, though its exact structure and the more active hormone T3 weren't characterized until decades later.

Thyroxine was isolated from thyroid tissue in the early 20th century. Synthetic levothyroxine entered routine use in the 1940s and largely replaced desiccated thyroid in mainstream endocrinology. See NDT for the glandular form.

Structure/Chemical properties

T4 is L-thyroxine (C₁₅H₁₁I₄NO₄): a tyrosine backbone with four iodine atoms. In serum it binds to thyroxine-binding globulin and albumin; only a small free fraction is available for cell uptake and deiodination.

Function/Mechanism of Action

Deiodinases in liver, gut, and peripheral tissues convert T4 to active T3. Reverse T3 (rT3) is an inactive route when conversion is shunted under stress or illness.

An excess of thyroxine, in a tissue that doesn't convert it rapidly to T3, has an antithyroid action.

— Ray Peat

In one experiment, thyroxine added to brain slices suppressed respiration, while T3 supports oxidative metabolism. Poor liver conversion, high Estrogen, and stress are common themes when T4-only therapy feels inadequate. See Hypothyroidism, Liver.

T4 has to be activated in the liver by removal of one iodine, and the antagonism between estrogen and thyroid slows the liver, so in women with low blood sugar or high estrogen symptoms T4 often fails to help or makes things worse. Cortisol and adrenaline under stress can shift T4 metabolism toward reverse T3, and selenium is the key nutrient for conversion.

T4 normally suppresses TSH because the pituitary converts it to T3 locally; enough T3 reaching the brain and pituitary does the same job, while reverse T3 competes for the active hormone's site.

The brain takes up and concentrates T3 while tending to expel T4, and raised T4 can lead to reverse T3 production inside the brain.

Medical uses/Effects (section 2)

Levothyroxine (synthetic T4) is standard prescription therapy for hypothyroidism worldwide. Mainstream care titrates dose by TSH. Broda Barnes and Ray Peat called for temperature, pulse, cholesterol, and symptoms alongside or instead of isolated TSH rules.

Oral T4 absorption depends on adequate stomach acid; Helicobacter pylori infection lowers stomach acid and can impair it. In a study of patients who stayed hypothyroid despite high-dose T4, eradicating H. pylori dropped mean TSH from 30.5 to 4.2 mIU/mL and raised free T3 and free T4 into normal range, with 21% of cases overshooting into transient thyrotoxicosis as absorption normalized. Persistently high TSH despite a high T4 dose is a signal to check for H. pylori or other malabsorption rather than simply raising the dose further; this mechanism is specific to oral T4 (and NDT, which is mostly T4) since direct T3 bypasses the stomach-acid-dependent absorption step entirely. Vitamin C both helps prevent H. pylori infection and improves eradication-therapy success rates.

Swallowing the T4 tablet with vitamin C lowered TSH by about 69% on average in two small studies without placebo, one in 31 poorly controlled patients with gastritis or other gastrointestinal disease (500 mg) and one in 28 patients needing high doses without known gut disease (1 g). See Hypothyroidism.

In a placebo-controlled trial in bipolar depression, levothyroxine doses above normal replacement added to existing treatment for 6 weeks lowered depression scores, and the drop tracked reduced glucose metabolism in the thalamus, dorsal striatum, and part of the cingulate cortex.

Side/Adverse effects (section 2)

Taken together with levothyroxine, ciprofloxacin cut T4 absorption by 39% while rifampin raised it by 25% in a crossover study.

Overtreatment causes hyperthyroid symptoms (tachycardia, anxiety, weight loss). Excess T4 without adequate conversion to T3 can worsen hypothyroid symptoms, including depression and low energy, until active T3 is supplied.

See also

• Mitochondria
• Wilson temperature syndrome