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Prescription medication — not a dietary supplement
T3 (Liothyronine)is a prescription (or investigational) drug, not a supplement. It is included here for reference because people research and discuss it (often used off-label) — not as a recommendation. Take it only under a qualified clinician's supervision and only as prescribed; do not source it from grey-market vendors, where identity, purity, and dosing are unverified. The evidence below reflects its clinical trials.
What the evidence says
Most T3 (Liothyronine) studies are mechanism or observational rather than RCTs that measure a clinical effect — keep findings provisional.
Most evidence is from medium-quality meta-analyses and randomised trials published 1984–2025 with a typical study size of 12 participants.
Based on 7 studies · 3 meta-analyses · 1 RCT · 2,025 total participants
Confidence
HighBy outcome
T3 (Liothyronine) has an evidence score of 3.4/10 — emerging evidence based on 7 indexed studies, including 3 meta-analyses. Synthetic triiodothyronine — the active thyroid hormone — approved as a prescription drug (Cytomel) for hypothyroidism. It genuinely raises basal metabolic rate, which is why bodybuilders take it off-label for fat loss and 'cutting.' But the honest story is the trade-off: supraphysiologic T3 burns lean body mass alongside fat (the extra weight lost is muscle, not fat), drives the heart into tachycardia and atrial-fibrillation risk, accelerates bone loss, and suppresses your own thyroid axis. It is NOT a dietary supplement, NOT a longevity drug, and there is no evidence it is a safe or effective physique drug. Low score, framed as harm reduction. Representative study: PMID 40795305.
The commonly studied dose of T3 (Liothyronine) is There is no endorsed off-label dose for fat loss or physique use — that use is unsupported and risky, and this library does not provide a cutting protocol. T3 is a prescription drug; approved hypothyroidism dosing is individualized by a clinician (typically initiated at 5 mcg/day and titrated against TSH and symptoms). Any use should be clinician-directed with thyroid-function and cardiac monitoring.. Individual needs vary — start at the lower end of the range and adjust based on how you respond.
Tesofensine
Mostly mechanism / observationalAn INVESTIGATIONAL triple monoamine (noradrenaline / dopamine / serotonin) reuptake inhibitor developed for obesity. Originally trialled for Parkinson's and Alzheimer's disease, it failed there but produced unexpectedly large weight loss — and its phase-2 TIPO-1 trial (Astrup, Lancet 2008) delivered roughly twice the weight loss of the drugs approved at the time (~9-11% at 0.5-1.0 mg). Honest appraisal: it is NOT approved in any major market, the headline efficacy rests mainly on phase-2 data, the pivotal TIPO-1 paper carries a journal Expression of Concern, and the centrally-acting monoaminergic mechanism brings a real cardiovascular (heart-rate) and psychiatric (mood/anxiety, insomnia) trade-off. NOT a dietary supplement and NOT a longevity drug — listed for reference only.
Last reviewed June 2026 · evidence from 7 studies · how we score
This information is for educational purposes only. It is not a substitute for professional medical advice. Always consult a qualified healthcare provider before starting, stopping, or changing any supplement or medication.
T3 (Liothyronine, Cytomel) — synthetic triiodothyronine
Synthetic triiodothyronine — the active thyroid hormone — approved as a prescription drug (Cytomel) for hypothyroidism. It genuinely raises basal metabolic rate, which is why bodybuilders take it off-label for fat loss and 'cutting.' But the honest story is the trade-off: supraphysiologic T3 burns lean body mass alongside fat (the extra weight lost is muscle, not fat), drives the heart into tachycardia and atrial-fibrillation risk, accelerates bone loss, and suppresses your own thyroid axis. It is NOT a dietary supplement, NOT a longevity drug, and there is no evidence it is a safe or effective physique drug. Low score, framed as harm reduction.
T3 (liothyronine) is an approved hypothyroidism drug that genuinely raises basal metabolic rate, but its off-label use for fat loss / 'cutting' is the framing here, and that use is poorly supported and risky. Supraphysiologic T3 makes the extra weight lost come from lean body mass rather than fat, is an established risk factor for atrial fibrillation and tachycardia, lowers bone mineral density when TSH is suppressed, and suppresses the endogenous thyroid axis. It is a prescription drug, not a supplement, and there is no evidence it is a safe or effective physique drug — hence the low score.
Liothyronine is synthetic L-triiodothyronine (T3), the biologically active thyroid hormone that the body normally makes by converting thyroxine (T4).
It is an approved prescription medicine (brand Cytomel) with a narrow, legitimate role: treating hypothyroidism — usually as second-line or add-on therapy when levothyroxine (T4) alone leaves a patient symptomatic — and short-term use in thyroid-cancer care and myxedema coma.
T3 binds nuclear thyroid-hormone receptors and upregulates genes that drive mitochondrial oxidative metabolism, raising basal metabolic rate and thermogenesis.
That metabolic effect is real and reproducible, and it is exactly why T3 has a following among bodybuilders and physique competitors, who take it off-label during 'cutting' phases to accelerate fat loss.
The reason this entry scores low and is framed as harm reduction is the gap between that mechanism and any safe, effective physique benefit.
First, body composition: when supraphysiologic T3 is added to a calorie-restricted diet, the extra weight lost comes disproportionately from lean body mass, not fat (Wolman 1985 found the extra weight loss was 'at the expense of lean body mass and not of fat'), and the body's adaptive fall in T3 during dieting is what normally protects muscle (Yang & van Itallie 1984).
For a physique user trying to preserve muscle, that is the opposite of the goal.
Second, the cardiac cost: T3 is a direct cardiac stimulant, and a low/suppressed TSH (the hallmark of supraphysiologic thyroid hormone) is an established risk factor for atrial fibrillation — the Framingham cohort found a threefold higher 10-year AF risk in older adults with suppressed thyrotropin (Sawin 1994) — alongside tachycardia, palpitations, and arrhythmia.
Third, bone: sustained TSH suppression lowers bone mineral density and is associated with osteoporosis, especially in postmenopausal women (Kwak 2021 meta-analysis). Fourth, the axis: exogenous T3 suppresses endogenous TSH and the hypothalamic-pituitary-thyroid axis, so the misuse is self-perpetuating.
In approved, regulated, monitored use, T3 (alone or as T4+T3 combination therapy) appears reassuringly safe — combination therapy gives a small subjective preference but no clear objective advantage over T4 monotherapy (Nassar 2024), and regulated liothyronine use is not associated with excess death or serious adverse events (Bahl 2025); the adverse events cluster in unregulated and compounding-error use.
The honest summary: T3 is a real hormone and a real medicine for hypothyroidism, it really does raise metabolic rate, but using it off-label as a fat-loss/physique drug trades fat for muscle and stacks cardiac, arrhythmic, and bone-loss risk on top of axis suppression — with no evidence it is a safe or effective way to get lean.
It is a prescription drug, not a supplement, and not a longevity compound.
T3 is the active thyroid hormone. It binds nuclear thyroid-hormone receptors (TRα/TRβ) and upregulates transcription of genes governing mitochondrial oxidative metabolism, Na/K-ATPase activity, and substrate turnover across virtually every tissue.
By driving oxidative metabolism, T3 raises basal metabolic rate and heat production. This thermogenic effect is the real, reproducible basis for the off-label fat-loss / 'cutting' use — but it is not selective for fat.
The same systemic catabolic, cardiac-stimulant, and bone-resorptive actions mean supraphysiologic T3 burns lean body mass alongside fat, accelerates heart rate and atrial-fibrillation risk, and lowers bone mineral density — and exogenous T3 suppresses the body's own TSH/thyroid axis.
How T3 (Liothyronine) works — from molecular targets to health outcomes. Click an edge to see supporting research.This visualization is in beta — pathways are being refined and expanded.
Tap node to isolate • Pinch to zoom • Tap edge for research
There is no endorsed off-label dose for fat loss or physique use — that use is unsupported and risky, and this library does not provide a cutting protocol. T3 is a prescription drug; approved hypothyroidism dosing is individualized by a clinician (typically initiated at 5 mcg/day and titrated against TSH and symptoms). Any use should be clinician-directed with thyroid-function and cardiac monitoring.
Can be taken without food
| Form | Type |
|---|---|
| 💊Oral tablet (liothyronine sodium) — prescription only, for hypothyroidism under a clinician | Recommended |
| 💊Levothyroxine (T4) monotherapy is first-line for hypothyroidism; desiccated thyroid and compounded T3 are not preferred | Alternative |
T3 is a prescription thyroid hormone, not a dietary supplement and not a longevity compound. There is no legitimate over-the-counter physique form.
Minimum: 2 weeks
Optimal: 12 weeks
Cycling: Not required
Note: When prescribed for hypothyroidism, taken in the morning, separated from food/calcium/iron. There is no endorsed physique-use timing — the off-label fat-loss use is unsupported and risky, and this library does not schedule it.
Dose-response data unavailable. The current published research for T3 (Liothyronine) does not provide sufficient dose-specific outcome data to generate reliable dose-response curves.
Refer to the Dosage & Timing section above for recommended dose ranges based on available evidence.
T3 genuinely increases basal metabolic rate and thermogenesis — the real mechanism behind the off-label fat-loss reputation.
As a prescription drug, T3 corrects thyroid-hormone deficiency — alone, in T4+T3 combination, or in thyroid-cancer care. This is its only evidence-based indication.
When supraphysiologic T3 accelerates weight loss on a diet, the extra weight comes disproportionately from lean body mass rather than fat — the opposite of a physique goal.
T3 stimulates the heart directly; a suppressed TSH is an established risk factor for atrial fibrillation, alongside tachycardia and palpitations.
Sustained TSH suppression lowers bone mineral density and risks osteoporosis; exogenous T3 also suppresses the body's own TSH/thyroid axis.
Avoid — off-label use for cutting burns lean mass and stacks cardiac, arrhythmia, and bone-loss risk with no evidence of a safe, effective physique benefit.
High risk — T3 is a cardiac stimulant and suppressed TSH raises atrial-fibrillation risk. Use only if prescribed, at the lowest effective dose, with monitoring.
Caution — TSH-suppressive thyroid-hormone exposure accelerates bone-mineral-density loss.
Manage hypothyroidism in pregnancy with a clinician (levothyroxine is preferred); do not use T3 off-label for weight.
Thyroid hormone potentiates anticoagulant effect — bleeding risk; dose adjustment and monitoring needed.
Additive cardiac stimulation — tachycardia and arrhythmia risk; a common and dangerous physique-use stack.
Tip: Dose-dependent cardiac stimulation; supraphysiologic dosing raises the risk. Reduce dose / seek care for sustained fast or irregular heartbeat.
Tip: When used for fat loss, the extra weight lost is disproportionately muscle, not fat — the opposite of a physique goal. No reliable mitigation at supraphysiologic doses.
Tip: A suppressed TSH is an established AF risk factor, especially in older adults; avoid suppressive dosing and monitor cardiac rhythm.
Tip: Sustained TSH suppression lowers bone mineral density, especially in postmenopausal women; avoid suppressive long-term dosing and monitor BMD.
The best time to take T3 (Liothyronine) is in the morning. It can be taken on an empty stomach. When prescribed for hypothyroidism, T3 is taken in the morning; its short half-life can mean divided dosing.
T3 (Liothyronine) should be used with caution — talk to a healthcare provider before taking it. The most commonly reported side effects are tachycardia / palpitations, loss of lean body mass, atrial fibrillation. Use caution if any of these apply to you: Uncorrected adrenal insufficiency (can precipitate adrenal crisis); Untreated thyrotoxicosis / hyperthyroidism; Cardiovascular disease, arrhythmia, or recent myocardial infarction (cardiac-stimulant risk).
Bimagrumab
Mostly mechanism / observationalAn investigational monoclonal antibody that blocks the activin type II receptor (ActRII) — the receptor through which myostatin and activin restrain muscle — producing a striking body recomposition (fat DOWN, lean muscle UP) but NO proven functional benefit. In a phase 2 trial in adults with type 2 diabetes and obesity, bimagrumab caused a large loss of fat mass and a real gain in lean mass over 48 weeks, which is why it is now being developed as a metabolic / obesity drug. But the muscle-disease story is a cautionary one: its pivotal phase 2b trial in inclusion-body myositis (RESILIENT) added muscle yet FAILED its primary walking endpoint, and a 2025 systematic review concluded it had no effect on disease progression. It is NOT an approved drug, NOT a dietary supplement, and NOT a longevity drug — gaining muscle on a scan is not the same as moving or living better. This entry exists to inform, not to recommend.
Thyroid hormone can raise glucose and alter diabetic control, changing insulin/oral-agent requirements.