Thyroid hormones (T3/T4)
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 action | 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-life | ~24 hours (steady-state within 7 days) | ~7 days (steady-state ~6 weeks) |
| Onset | Metabolic-rate increase 24-48 hours, fat-loss effect 1-2 weeks | Metabolic-rate increase 1-2 weeks; steady-state 6 weeks |
| Legal status | 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 action
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-life
Liothyronine (T3, Cytomel)~24 hours (steady-state within 7 days)
Levothyroxine (T4, Synthroid)~7 days (steady-state ~6 weeks)
Onset
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 status
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.