Thyroid hormones (T3/T4)Thyroid hormone (T3/T4)The thyroid’s hormones: T4 (a storage prohormone) and the active T3, which regulate the body’s baseline metabolic rate.
Liothyronine vs Levothyroxine
The table compares the compounds’ key data from their entry pages. Open each compound’s full entry for details. Educational content, not medical advice.
 Liothyronine (T3, Cytomel) | Levothyroxine (T4, Synthroid) | |
|---|---|---|
| What it is | Synthetic T3 (3,5,3'-triiodothyronine), Pfizer 1956 FDA-approved hypothyroidism Rx (Cytomel). Cutting-cycle off-label thermogenic +10-30% REE via direct mitochondrial β-oxidation + Na/K-ATPase upregulation; WADA in-competition banned for non-Rx athletes. | Synthetic T4 (3,5,3',5'-tetraiodothyronine), Knoll/AbbVie 1955 FDA-approved hypothyroidism Rx gold standard (Synthroid). A prohormone – hepatic and renal 5'-deiodinase (D1/D2) convert it to active T3. Off-label cutting-cycle use is less common than T3 (slower 7-day t1/2, steady-state ~6 weeks). WADA: allowed with documented hypothyroidism diagnosis. |
| Mechanism of actionMechanism of actionThe molecular pathway by which a compound acts in the body (e.g. which receptor or enzyme it binds). | Nuclear TR-α + TR-β receptor agonist, gene-transcription activation, +10-30% REE | Thyroid prohormone, peripheral D1/D2 conversion to T3, indirect nuclear TR-α/β agonist |
| Dosing | Clinical 25-50 μg/day; off-label cutting-cycle 25-75 μg/day, titration 12.5 μg/step | Clinical 1.6 μg/kg/day (100-150 μg/day); off-label cutting 200-300 μg/day (NOT recommended) |
| Half-lifeHalf-lifeThe time for the compound’s plasma concentration to fall by half; indicates how fast it clears and how often it is dosed. | ~24 hours (steady-state within 7 days) | ~7 days (steady-state ~6 weeks) |
| OnsetOnsetThe time from dosing until a measurable effect appears. | Metabolic-rate increase 24-48 hours, fat-loss effect 1-2 weeks | Metabolic-rate increase 1-2 weeks; steady-state 6 weeks |
| Legal statusLegal / WADA statusThe compound’s regulatory status (e.g. prescription) and its place on the WADA prohibited-in-sport list. | FDA + EMA Rx (hypothyroidism), registered in HU + PL. WADA: S4.5 in-competition banned for non-Rx athletes. | FDA + EMA Rx (hypothyroidism), registered in HU + PL. WADA: allowed with documented hypothyroidism diagnosis, off-label cutting is NOT legitimate. |
| Full entry | Open → | Open → |
What it is
Liothyronine (T3, Cytomel)Synthetic T3 (3,5,3'-triiodothyronine), Pfizer 1956 FDA-approved hypothyroidism Rx (Cytomel). Cutting-cycle off-label thermogenic +10-30% REE via direct mitochondrial β-oxidation + Na/K-ATPase upregulation; WADA in-competition banned for non-Rx athletes.
Levothyroxine (T4, Synthroid)Synthetic T4 (3,5,3',5'-tetraiodothyronine), Knoll/AbbVie 1955 FDA-approved hypothyroidism Rx gold standard (Synthroid). A prohormone – hepatic and renal 5'-deiodinase (D1/D2) convert it to active T3. Off-label cutting-cycle use is less common than T3 (slower 7-day t1/2, steady-state ~6 weeks). WADA: allowed with documented hypothyroidism diagnosis.
Mechanism of actionMechanism of actionThe molecular pathway by which a compound acts in the body (e.g. which receptor or enzyme it binds).
Liothyronine (T3, Cytomel)Nuclear TR-α + TR-β receptor agonist, gene-transcription activation, +10-30% REE
Levothyroxine (T4, Synthroid)Thyroid prohormone, peripheral D1/D2 conversion to T3, indirect nuclear TR-α/β agonist
Dosing
Liothyronine (T3, Cytomel)Clinical 25-50 μg/day; off-label cutting-cycle 25-75 μg/day, titration 12.5 μg/step
Levothyroxine (T4, Synthroid)Clinical 1.6 μg/kg/day (100-150 μg/day); off-label cutting 200-300 μg/day (NOT recommended)
Half-lifeHalf-lifeThe time for the compound’s plasma concentration to fall by half; indicates how fast it clears and how often it is dosed.
Liothyronine (T3, Cytomel)~24 hours (steady-state within 7 days)
Levothyroxine (T4, Synthroid)~7 days (steady-state ~6 weeks)
OnsetOnsetThe time from dosing until a measurable effect appears.
Liothyronine (T3, Cytomel)Metabolic-rate increase 24-48 hours, fat-loss effect 1-2 weeks
Levothyroxine (T4, Synthroid)Metabolic-rate increase 1-2 weeks; steady-state 6 weeks
Legal statusLegal / WADA statusThe compound’s regulatory status (e.g. prescription) and its place on the WADA prohibited-in-sport list.
Liothyronine (T3, Cytomel)FDA + EMA Rx (hypothyroidism), registered in HU + PL. WADA: S4.5 in-competition banned for non-Rx athletes.
Levothyroxine (T4, Synthroid)FDA + EMA Rx (hypothyroidism), registered in HU + PL. WADA: allowed with documented hypothyroidism diagnosis, off-label cutting is NOT legitimate.