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BHuman cohort / observational
CardiovascularCardiotoxicHigh risk

T3

Cytomel · Liothyronine

T3 (liothyronine, Cytomel) is the synthetic form of triiodothyronine, the biologically active thyroid hormone. Medically it is used for hypothyroidism, myxedema coma, and as part of investigational levothyroxine/liothyronine combination therapy. Outside medicine it is misused to accelerate fat loss and raise metabolic rate. The main danger is that any dose meaningfully above physiologic replacement produces a state of iatrogenic thyrotoxicosis (hormone excess), and T3 is more potent and faster-acting than T4, making overshoot easy. Documented harms of thyroid-hormone excess include a roughly threefold increase in atrial fibrillation risk, increased cardiovascular mortality, accelerated bone loss with higher fracture risk, and, in extreme excess, life-threatening thyroid storm (multi-organ failure, ~11% mortality in hospitalized series). During a caloric deficit the body deliberately lowers its own T3 to spare protein; overriding that with exogenous T3 is expected to increase loss of lean/muscle mass, not just fat. There are no human trials evaluating T3 as a physique or performance drug, so that use is unstudied and the harms below are extrapolated from human data on thyroid-hormone excess. This is not dosing guidance; it is a risk profile. Anyone using or considering T3 should be under the care of a physician with thyroid function monitoring.

Clinical readoutPED-adjacent · thyroid-hormone
Hepatic strainModerate
CardiovascularHigh
HPTA suppressionNone
Half-life
24 h
Route
Oral
Evidence
B
Active
Biological/metabolic ef…
24 h2 d3 d4 d5 d
Illustrative single-compartment washout · each mark = one half-life · t½ ≈ Short relative to T4. Human kinetic studies characterize T3 turnover and metabolic clearance; plasma half-life is commonly cited at roughly 1 day (about 0.75-1.5 days) in euthyroid adults, prolonged in hypothyroidism and shortened in hyperthyroidism. Exact value depends on thyroid status and assay method.
Pharmacology

Mechanism of action

Triiodothyronine is the active thyroid hormone. It enters cells and binds nuclear thyroid hormone receptors (TRalpha and TRbeta), which act as ligand-dependent transcription factors regulating genes governing basal metabolic rate, thermogenesis, cardiac chronotropy and inotropy, lipid and carbohydrate metabolism, and bone turnover. T4 (thyroxine) is largely a prohormone that is peripherally deiodinated to T3; administering T3 directly bypasses this regulated conversion step, producing faster onset and less physiologic buffering. Supraphysiologic T3 exposure upregulates metabolic rate and beta-adrenergic sensitivity (tachycardia, tremor) and, via negative feedback on the hypothalamic-pituitary-thyroid axis, suppresses TSH and endogenous thyroid hormone production.
Kinetics

Pharmacokinetics

Half-life

Short relative to T4. Human kinetic studies characterize T3 turnover and metabolic clearance; plasma half-life is commonly cited at roughly 1 day (about 0.75-1.5 days) in euthyroid adults, prolonged in hypothyroidism and shortened in hyperthyroidism. Exact value depends on thyroid status and assay method.

Active duration

Biological/metabolic effects outlast plasma levels because the drug acts through gene transcription; clinical effects persist for days after a dose and after discontinuation. Onset is rapid (hours).

Route

Oral (liothyronine sodium tablets); an intravenous form exists for myxedema coma in hospital settings.

Metabolism & clearance

Cleared by hepatic and peripheral deiodination (sequential removal of iodine), glucuronide/sulfate conjugation, and biliary/renal excretion of metabolites. Metabolic clearance rate and production rate have been quantified in human radiotracer studies. Note: pharmacokinetic detail is provided for monitoring, washout, and clinician assessment, not for evading detection.

For monitoring and washout planning, not drug-test evasion.

Reported effects

Physiological & performance effects

  • Increases basal metabolic rate and thermogenesis (raised heat production, sweating, heat intolerance)
  • Increases heart rate and cardiac contractility; can cause palpitations and raise systolic blood pressure
  • During caloric restriction, endogenous T3 normally falls as a protein-sparing adaptation; supplying exogenous T3 opposes this adaptation and is expected to increase catabolism of lean/muscle protein alongside fat
  • Suppresses pituitary TSH and endogenous thyroid hormone output via negative feedback
  • Investigational addition to levothyroxine has not shown consistent symptomatic benefit over levothyroxine alone across 14 randomized trials
  • At supraphysiologic exposure produces the clinical picture of thyrotoxicosis: tremor, anxiety, weight loss, tachyarrhythmia, muscle weakness
Safety

Adverse effects by system

Cardiovascular

Most serious system. Thyroid hormone excess (including a suppressed TSH without overt symptoms) is associated with roughly a threefold increased risk of atrial fibrillation over 10 years in older adults, and with increased cardiovascular mortality in overt and subclinical hyperthyroidism. Also causes sinus tachycardia, palpitations, increased myocardial oxygen demand (can precipitate angina/ischemia in those with coronary disease), and can worsen heart failure. Severe excess can trigger thyroid storm with acute heart failure.

Hepatic

Liver function test abnormalities occur in hyperthyroidism/thyroid-hormone excess, attributed to oxidative stress, relative hepatic hypoxia, cholestasis, and increased osteoblastic alkaline phosphatase. Usually mild and reversible with normalization of thyroid status; T3 itself is not a classic direct hepatotoxin.

Endocrine / HPTA

Exogenous T3 suppresses the hypothalamic-pituitary-thyroid axis: TSH falls and endogenous thyroid hormone production is downregulated. After stopping, the axis is generally expected to recover, but the trajectory and any need for support should be assessed by an endocrinologist rather than self-managed.

Reproductive

Thyroid hormone excess can disturb reproductive function (menstrual irregularity in women; altered sex-hormone-binding globulin and possible effects on libido/fertility), but a specific primary human source quantifying reproductive harm from exogenous T3 was not retrieved. State as limited/no adequate data and defer to a clinician.

Neuropsychiatric

Excess thyroid hormone can produce anxiety, irritability, restlessness, insomnia, and, in severe thyrotoxicosis/thyroid storm, agitation, delirium, and altered mental status. Dedicated controlled data on psychiatric effects of T3 misuse specifically are lacking.

Renal

No well-established direct nephrotoxicity from T3 in the retrieved human literature. Thyroid status alters renal blood flow and glomerular filtration and can shift creatinine-based estimates, but a specific primary source for renal injury from T3 misuse was not identified; state as limited/no adequate data.

Hematologic

No specific primary source for direct hematologic toxicity from T3 was identified in this search. General thyrotoxicosis literature notes possible mild changes; treat as limited/no adequate data.

Dermatologic

Thyrotoxicosis is associated with warm, moist skin, sweating, and heat intolerance; hair changes can occur. No specific primary source for serious dermatologic injury from T3 misuse was identified beyond the general thyrotoxic picture.

Recovery

HPTA suppression & recovery

Suppression: Applies to the thyroid (HPT) axis rather than the gonadal axis: exogenous T3 suppresses TSH and endogenous thyroid hormone output; degree scales with dose and duration. T3 is not androgenic and does not act on the HPG axis the way anabolic steroids do.

The pituitary-thyroid axis is generally expected to recover after discontinuation, but recovery time varies and cannot be assumed. This is conservative, general information only: assessment and any management of axis recovery should be directed by an endocrinologist with thyroid function testing, not self-managed. (No SERM-based intervention is relevant to thyroid-axis recovery.)

Bloodwork & vitals

Monitoring

Recommended labs & checks
TSHFree T3 and Free T4Resting heart rate and blood pressureECG if palpitations, irregular pulse, or age/CV risk (screen for atrial fibrillation)Liver function tests if symptomatic or baseline abnormalBone mineral density (DXA) for prolonged use or in those at fracture risk

Cadence: Under physician care, thyroid function is typically checked before starting and roughly 4-6 weeks after any change, then periodically once stable; cardiovascular symptoms warrant prompt reassessment. Any non-medical use should prompt a clinician visit rather than self-directed lab timing.

Warning signs — seek care
  • Palpitations, irregular or racing heartbeat (possible atrial fibrillation) - seek urgent care
  • Chest pain or shortness of breath
  • Fever, agitation, confusion, vomiting/diarrhea, or extreme tachycardia (possible thyroid storm - medical emergency)
  • Unintended rapid weight loss, severe tremor, heat intolerance, insomnia, anxiety
  • New bone pain or fragility fracture
  • Any signs of hyperthyroidism - stop and consult a physician
Do not use if

Contraindications

  • Untreated thyrotoxicosis / pre-existing hyperthyroidism
  • Uncorrected adrenal insufficiency (can precipitate adrenal crisis; thyroid hormone increases cortisol clearance)
  • Recent myocardial infarction or acute coronary syndrome; known coronary artery disease or angina (increases myocardial oxygen demand)
  • Atrial fibrillation or other tachyarrhythmia, and uncontrolled cardiovascular disease
  • Heart failure
  • Use for weight loss or physique/performance goals in the absence of thyroid disease (no supporting human trial evidence; harm-forward)
  • Osteoporosis or high fracture risk (accelerated bone loss)
  • Elderly patients and any use without physician supervision and thyroid function monitoring
Combinations

Interaction profile

  • MajorWith an anabolic steroid: Additive cardiovascular strain
  • ModerateWith an anabolic steroid: Additive cardiovascular strain
  • MajorWith a stimulant: Additive cardiovascular strain
  • ModerateWith a SARM: Additive cardiovascular strain
  • MajorWith a thermogenic stimulant: Additive cardiovascular strain
  • ModerateWith a GLP-1 / incretin agonist: Additive cardiovascular strain
  • ModerateWith a melanocortin agonist: Additive hypertension
  • MajorWith a QT-prolonging drug: QT prolongation
  • ContraindicatedWith DNP: Additive cardiovascular strain

Check a specific combination in the interaction checker.

Harm reduction

Reducing harm & when to stop

  • Non-medical use of T3 for fat loss or performance is unstudied in humans and carries cardiovascular and skeletal risks that are well-documented for thyroid-hormone excess; the safest option is not to use it without a diagnosed medical need.
  • If used, it should be under a physician/endocrinologist with baseline and follow-up thyroid function tests (TSH, free T3, free T4).
  • Stop and seek urgent medical care for palpitations, an irregular or racing pulse, chest pain, or shortness of breath (possible atrial fibrillation or cardiac strain).
  • Fever, agitation, confusion, vomiting, and extreme tachycardia can indicate thyroid storm, a medical emergency requiring immediate hospital care.
  • Do not combine with stimulants or other agents that raise heart rate; this compounds arrhythmia and cardiac risk.
  • People with heart disease, arrhythmia, osteoporosis, adrenal insufficiency, or who are elderly are at especially high risk and should not self-administer.
  • During dieting, exogenous T3 is expected to increase loss of muscle/lean mass, not selectively fat; this undermines the usual goal and adds catabolic risk.
  • Report all use to your clinician so cardiac and bone health can be monitored; do not treat thyroid axis recovery as automatic.
Evidence

Citations (11)

Every clinical claim above is tied to a primary source. Overall evidence grade B — graded to the best available evidence for its core claims.

  1. 01

    T3/liothyronine is used medically for hypothyroidism and has been studied as an addition to levothyroxine, without consistent benefit over levothyroxine alone across 14 randomized trials.

    GuidelinePMID 33777817 ↗

  2. 02

    Low serum TSH (thyroid-hormone excess) is associated with an approximately threefold increased risk of atrial fibrillation over 10 years in older adults.

    CohortPMID 7935681 ↗

  3. 03

    Low serum TSH per se is a risk factor for atrial fibrillation and possibly other cardiovascular disease in subclinical hyperthyroidism.

    ReviewPMID 12165113 ↗

  4. 04

    Overt and subclinical hyperthyroidism, particularly with undetectable TSH, may increase cardiovascular mortality.

    Meta-analysisPMID 22802423 ↗

  5. 05

    Thyrotoxicosis (endogenous or exogenous) accelerates bone remodeling and is a risk factor for osteoporosis and fracture; bone loss partially reverses with treatment.

    ReviewPMID 22561612 ↗

  6. 06

    Liver function test abnormalities occur in hyperthyroidism due to oxidative stress, cholestasis, and increased osteoblastic activity, and generally reverse with normalized thyroid status.

    ReviewPMID 32166702 ↗

  7. 07

    Severe thyroid-hormone excess can cause thyroid storm, characterized by multi-organ failure and acute heart failure, with mortality around 11% in a large national series.

    GuidelinePMID 27746415 ↗

  8. 08

    During caloric restriction, endogenous serum T3 falls (with a rise in reverse T3), an adaptation associated with conservation of visceral and muscle protein; implying that supplying exogenous T3 during a deficit opposes protein-sparing.

    CohortPMID 3978824 ↗

  9. 09

    Very-low-calorie dieting lowers T3 and free T3 and reduces metabolic rate, part of the adaptive response to energy deficit.

    RCTPMID 3189211 ↗

  10. 10

    Human kinetic studies quantify T3 production rate, metabolic clearance rate, and turnover, which are reduced in hypothyroid states and altered by thyroid status.

    CohortPMID 6797765 ↗

  11. 11

    T3 and T4 metabolic clearance and production rates are directly measurable in humans: in a single case report of a patient with pituitary/peripheral thyroid-hormone RESISTANCE syndrome (elevated T3/T4 with a normal TSH, no Graves' disease or goiter — mechanistically distinct from ordinary thyrotoxicosis), both the metabolic clearance rate and production rate of T3 and T4 were increased. This single atypical case demonstrates that such kinetics can be quantified in humans but does not by itself establish the clearance/production time-course for ordinary states of thyroid-hormone excess.

    Case reportPMID 113140 ↗

Last reviewed 2026-07-06 · Verified against PubMed · Educational, not medical advice