Triiodothyronine

Triiodothyronine
Triiodothyronine (T3) molecular structure
Abbreviation	T3
Molecular formula	C₁₅H₁₂I₃NO₄
Type	Thyroid hormone (active form)
Administration	Oral (tablet), intravenous
Bioavailability	~95% orally
Synonyms	Liothyronine, T3, 3,5,3'-triiodo-L-thyronine, Cytomel
Source	Endogenous: thyroid gland (~20% direct secretion); peripheral conversion from T4 in liver and other tissues
Ray's verdict	Generally helps Essential/Positive: T3 is the metabolically active thyroid hormone. Direct T3 bypasses impaired liver conversion of T4, common with high Estrogen, stress, or liver dysfunction. Single large bolus doses can trigger hepatic clearance enzymes and hypothyroid rebounds 10-12 hours later; gradual dosing with food is safer.

Triiodothyronine (T3, liothyronine) is the biologically active thyroid hormone that regulates metabolic rate and cellular respiration. The thyroid secretes roughly 80% T4 and 20% T3; most circulating T3 (~80%) is made outside the gland by deiodination of T4, chiefly in the Liver.

While T4 is often called "the thyroid hormone," T3 carries most calorigenic and mitochondrial effects. T3 is functionally the more important hormone, especially when liver conversion of T4 is poor.

History

Etymology

The name is descriptive: "tri-" (three) "iodo-" (iodine atoms) attached to "thyronine," the amino-acid-derived backbone shared with thyroxine.

Jack Gross and Rosalind Pitt-Rivers identified T3 in 1952, showing it was more biologically active than thyroxine despite circulating at much lower concentrations.

Iodine was linked to thyroid function by 1820. Thyroxine (T4) was isolated early in the 20th century; Rosalind Pitt-Rivers and colleagues identified T3 in the 1950s as a major active hormone. Natural desiccated thyroid (both T4 and T3) dominated hypothyroidism treatment for decades before synthetic levothyroxine. See NDT.

The name means three iodine atoms on a thyronine backbone; T4 has four.

Structure/Chemical properties

T3 is C₁₅H₁₂I₃NO₄: one fewer iodine than T4 (missing the 5' iodine on the outer ring), which accounts for its higher receptor affinity. In serum, ~0.3% circulates as free hormone; the rest binds transport proteins.

Function/Mechanism of Action

T3 drives the calorigenic (heat-producing) action of thyroid hormone in adult tissues. Adding T3 to tissue increases respiration, while T4 added to brain slices can suppress respiration in vitro.

Mitochondria respond directly to thyroid hormones; T3 increases mitochondrial number, size, and oxidative capacity. A 2026 review frames T3 as the top-level hormonal regulator of mitochondrial quality control, driving both biogenesis (via PGC-1α and NRF1/NRF2) and mitophagy (clearance of damaged mitochondria); age-related decline in T3 signaling impairs both processes, letting mutant mitochondrial DNA accumulate, a core driver of the mitochondrial-dysfunction hallmark of aging and a mechanistic link to cancer risk.

T3 supports glucose oxidation, Krebs cycle enzymes, and glucose transporters. Thyroid is the hormone of respiration because adequate T3 enables efficient CO₂ production from glucose.

The liver supplies about 70% of active thyroid hormone by converting T4 to T3, but only when it has adequate glucose. Frequent small carbohydrate snacks, such as sipping orange juice every hour, keep liver T3 output up; protein alone stimulates insulin but not T3 secretion.

Much of T3's calorigenic effect comes from Na⁺/K⁺-ATPase activation.

Cholesterol was used as a thyroid marker before it became a cardiovascular risk marker. T3 supports LDL clearance and conversion of cholesterol into steroid hormones and bile acids. See Thyroid and conversion.

Progesterone supports thyroxine secretion and, unlike the anti-thyroid action of lithium, appears to inhibit the formation of reverse T3.

Reverse T3 (rT3) is an inactive deiodination product. Under stress, illness, or excess T4 without adequate conversion, T4 can shunt to rT3 instead of active T3, producing functional hypothyroidism at the tissue level despite normal serum T4. Impaired T4/T3 membrane transport into peripheral cells is a separate mechanism producing the same picture: pituitary intracellular T3 stays adequate (keeping TSH "normal") while peripheral tissue is starved of hormone, so serum panels alone underdiagnose it. See Hypothyroidism.

The fact that the T3 is being used faster than T4, removing it from the blood more quickly than it enters from the thyroid gland itself, hasn't been discussed in the journals, possibly because it would support the view that a natural glandular balance was more appropriate to supplement than pure thyroxine.

— Ray Peat

Medical uses/Effects

Liothyronine (synthetic T3), NDT, and T4/T3 combinations treat Hypothyroidism when tissue T3 is inadequate. Female livers often convert T4 to T3 less efficiently than male livers, so T4-only therapy can feel inadequate or even antithyroid in some women.

TRIAC (triiodothyroacetic acid)

TRIAC is a natural T3 metabolite that retains some T3-like activity. In rats given inescapable shock to induce learned-helplessness-like escape deficits, daily TRIAC injections prevented those deficits as effectively as tricyclic antidepressants; a follow-up study found TRIAC hastened the antidepressant reversal of shock-induced escape deficits when combined with standard antidepressants. See Learned helplessness, Serotonin.

T3-containing therapy outperforms T4-only

In a retrospective comparison of hypothyroid patients dosed by symptoms rather than TSH, switching from levothyroxine to NDT (which supplies both T4 and T3) roughly doubled 24-hour urinary free T3 and cut symptom-score prevalence for constipation, headache, muscle cramps, depression, rheumatoid complaints, cold intolerance, and fatigue by 2.5- to 6.5-fold. Euthyroid Hashimoto's patients (normal TSH on T4-only treatment) still show measurable physical fatigability and elevated pain/fatigue perception versus controls despite unremarkable neuromuscular testing, consistent with normalized TSH not equaling normalized tissue T3. In children, varied rheumatic manifestations including fibromyalgia and arthralgias have resolved with thyroid replacement, indicating hypothyroidism's musculoskeletal presentation is not limited to adults.

T3 can normalize elevated cholesterol quickly by increasing conversion to Progesterone and bile acids.

In critically ill patients requiring non-surgical intensive care, low circulating free T3 (low-T3/non-thyroidal-illness syndrome) tracked overall illness severity and predicted worse outcomes across a broad range of critical conditions including heart failure, functioning as a general prognostic marker rather than a cardiovascular-specific one.

A NHANES cohort study of 3,603 adults over 50 (981 deaths) using Cox proportional-hazards models found TSH itself unrelated to mortality, but free T3 protective (hazard ratio 0.88 per quartile increase) and free T4 harmful (hazard ratio 1.26), both P<0.01 and robust after adjusting for socioeconomic status and adiposity, pointing at T3 specifically, not TSH or T4, as the hormone that tracks longevity.

In acute myxedema coma, intravenous T3 has succeeded when high-dose T4 failed or worsened the patient.

A controlled trial found T3 (as Cytomel) roughly doubled the rate of blood alcohol decline in acute alcohol intoxication compared to untreated controls, clearing patients faster than the body's normal metabolism alone. A declassified CIA MKUltra-era internal letter (c. 1961) proposed using this effect operationally: giving field agents Cytomel tablets so they could drink to stay sociable during an assignment and sober up within twenty to thirty minutes.

Broda Barnes linked adequate thyroid supplementation to lower cardiovascular mortality in his long clinical series.

Small bedtime T3 doses sometimes relieve menopausal insomnia and restore relaxation rate in calf and heart muscle.

Liver dysfunction impairs T4→T3 conversion; T3 supplementation can bypass that step when conversion is poor. Glucose and Selenium support the conversion enzyme.

Oral T4 absorption also depends on adequate stomach acid; Helicobacter pylori infection or other causes of low stomach acid can impair it specifically, producing persistently high TSH despite a high T4 dose. Direct T3 bypasses this absorption step entirely, since it does not rely on gastric acid for uptake. See Thyroxine for the eradication-study numbers.

Mainstream psychiatry has trialed T3 augmentation in treatment-resistant depression; restored oxidative metabolism is the more coherent framing rather than isolated serotonin pathways.

Dosing

Supplement protocols, starting doses, monitoring (temperature, pulse, labs), and warnings are on Roadmap/05 - Supplementing T3/T4. See also NDT, Thyroxine, Basal temperature, Roadmap/02 - Self diagnosis & markers, and Harm reduction.

The body produces only a few micrograms of T3 per hour; doses above that need careful titration. Taking more than ~10 mcg at once can signal the liver to upregulate T3-clearing enzymes, producing a hypothyroid rebound 10-12 hours later.

To approximate the body's own physiological rate of T3 formation, a 10-15 mcg Cytomel tablet can be nibbled through the day rather than swallowed whole; a large single dose is more likely to be converted to inactive reverse T3 as a normal liver defensive response. A piece of fruit, a glass of juice or milk between meals, and adequate protein can sometimes let the liver produce enough T3 on its own, without a T3 supplement at all.

Traditional equivalency: ~100 mcg T4 ≈ 25 mcg T3 ≈ one grain (60-65 mg) desiccated thyroid.

Side/Adverse effects

Thyroid gland suppression from supplementation is usually brief. High doses suppress endogenous output, but normal gland function returned within days after stopping, unlike adrenal suppression from Cortisol.

Hypothyroid people often have high compensatory Adrenaline, which can cause palpitations and elevated blood pressure; correcting T3 deficiency typically lowers these symptoms.

Overtreatment causes tachycardia, anxiety, and palpitations. Sensitive individuals may need concurrent Sugar and Magnesium support because thyroid helps cells retain magnesium and potassium.

Hyperthyroidism from thyroid hormone does not necessarily cause osteoporosis; hypothyroidism retards bone remodeling.

Unsaturated fats inhibit thyroid function at several steps: thyroglobulin proteolysis, hormone transport, T4→T3 conversion, and tissue response proportional to double bonds.

A 2025 systematic review and meta-analysis covering 52 studies (21 randomized trials, 4 cohort studies, 27 case reports), UK Yellow Card reports, and the US FDA adverse-event database (FAERS) found that adverse events attributed to liothyronine (T3) occurred almost exclusively with unregulated, self-sourced T3 or pharmacy compounding errors, not with properly prescribed and monitored use. T3 and T4 had similarly severe adverse-event profiles in the Yellow Card data, FAERS showed no excess T3 safety signal, combination T3+T4 therapy carried a similar adverse-event risk to T4 monotherapy in pooled trial data, and a pooled cohort analysis of 630,254 people found no increased risk of atrial fibrillation, heart failure, or stroke with T3 versus T4-only use, along with a lower all-cause mortality risk on T3.