Liothyronine (T3)
Also known as: T3, Cytomel, Triiodothyronine
Thyroid Hormone | Metabolic Optimization
Overview
Liothyronine (T3) is the biologically active form of thyroid hormone, responsible for regulating basal metabolic rate, thermogenesis, protein synthesis, and cellular energy metabolism throughout the body. Unlike levothyroxine (T4), which serves primarily as a prohormone requiring peripheral conversion by deiodinase enzymes, T3 acts directly on nuclear thyroid hormone receptors to exert its metabolic effects. It has been FDA-approved since the 1950s for the treatment of hypothyroidism, myxedema coma, and as a diagnostic agent in thyroid suppression tests. Pharmaceutical T3 is available as Cytomel (brand) and generic liothyronine sodium tablets. In performance and biohacking contexts, T3 is widely used during cutting phases to accelerate fat loss by directly upregulating metabolic rate, and by individuals seeking metabolic optimization when peripheral T4-to-T3 conversion is impaired due to caloric restriction, stress, or other factors.
Key benefits
- Direct and rapid increase in basal metabolic rate
- Enhanced fat oxidation and lipolysis
- Improved energy levels and reduction of hypothyroid fatigue
- Faster-acting than T4 with effects noticeable within hours to days
- Useful when peripheral T4-to-T3 conversion is impaired (illness, caloric deficit, stress)
- Supports body temperature regulation and thermogenesis
- Can improve cognitive clarity and reduce brain fog associated with low thyroid function
Mechanism of action
Liothyronine enters target cells via monocarboxylate transporter 8 (MCT8) and other thyroid hormone transporters, then binds to nuclear thyroid hormone receptors (primarily TR-beta in metabolic tissues and TR-alpha in cardiac tissue). The T3-receptor complex forms heterodimers with retinoid X receptors (RXR) and binds to thyroid hormone response elements (TREs) on DNA, directly modulating gene transcription. This increases expression of Na+/K+ ATPase (driving cellular energy expenditure), uncoupling proteins (UCP1 in brown adipose tissue, promoting thermogenesis), and enzymes involved in fatty acid oxidation, gluconeogenesis, and protein turnover. T3 also upregulates beta-adrenergic receptor density, which amplifies catecholamine sensitivity and contributes to increased heart rate, lipolysis, and thermogenesis. At physiological doses, T3 supports healthy metabolism; at supraphysiological doses, it shifts the body into a catabolic state where both fat and lean tissue can be catabolized for energy.
Molecular data
- Type
- Thyroid hormone (C15H12I3NO4)
- Half-life
- ~1 day
Indications
What the research community uses this compound for, with self-reported effectiveness.
Endocrine / Thyroid
Body Composition / Performance
Dosing protocols
Common protocols by delivery method. Adjust the curve below to model accumulation in your own cycle.
Delivery method
| Protocol | Dose | Frequency | Duration |
|---|---|---|---|
| — | 25-75 mcg/day | Once daily or split into 2-3 doses | — |
| — | 5-25 mcg/day | Once daily | — |
| — | 25-50 mcg/day | Split into 2 doses (AM and early PM) | — |
| — | 50-75 mcg/day | Split into 2-3 doses | — |
| — | 5-15 mcg/day (added to T4) | Once daily or split into 2 doses | — |
Safety
Reported adverse effects, contraindications, and what to monitor on cycle.
Common side effects
- Tachycardia and palpitations (increased heart rate, especially at higher doses)
- Anxiety, nervousness, and irritability
- Insomnia and disrupted sleep architecture
- Increased sweating and heat intolerance
- Tremor (fine hand tremor, similar to hyperthyroid presentation)
- Increased appetite despite accelerated metabolism
- Loose stools or increased bowel frequency
Rare side effects
- Muscle wasting and significant lean tissue loss at high doses (above 75 mcg/day)
- Bone mineral density loss with prolonged supraphysiological use
- Cardiac arrhythmias (atrial fibrillation at high doses or in predisposed individuals)
- Angina pectoris or exacerbation of existing coronary artery disease
- Adrenal crisis in patients with untreated adrenal insufficiency (T3 accelerates cortisol clearance)
- Menstrual irregularities in women
- Hair thinning or increased hair shedding
Contraindications
- Untreated adrenal insufficiency (must correct cortisol deficiency before starting T3)
- Acute myocardial infarction or unstable angina
- Thyrotoxicosis or untreated hyperthyroidism
- Known hypersensitivity to liothyronine or any tablet excipients
- Uncorrected adrenal cortical insufficiency (risk of adrenal crisis)
Monitoring
- TSH: baseline, 4-6 weeks after starting, then every 3-6 months (will be suppressed on exogenous T3)
- Free T3 and free T4: at each lab draw to assess thyroid hormone balance
- Resting heart rate and blood pressure: self-monitor daily during dose titration
- Body composition or DEXA: periodically to assess lean tissue preservation
- Bone mineral density (DEXA): annually for prolonged supraphysiological use
- Cortisol / adrenal function: baseline assessment before starting, especially in those with fatigue or suspected adrenal insufficiency
References
Primary literature and clinical-trial data informing this entry.