OnePin Peptide Library · Metabolic
T3 (Liothyronine / Cytomel)
T3 · Cytomel · Liothyronine Sodium
Liothyronine sodium (T3) is the synthetic form of the active thyroid hormone 3,5,3'-triiodothyronine. It is the biologically active form produced endogenously both by the thyroid gland directly and by peripheral deiodination of T4 (thyroxine) in the liver, kidneys, and other tissues. FDA-approved under the brand names Cytomel and Triostat, it is used for the treatment of hypothyroidism, myxedema coma (intravenous), and as a diagnostic suppression test for thyroid cancer.
Given the ~24 hour half-life, once-daily morning dosing is standard for replacement therapy. For higher off-label doses, splitting AM/PM smooths cardiac load. Always titrate up by 12.5 mcg increments every 3-5 days, monitoring resting heart rate, blood pressure, and symptoms (tremor, heat intolerance, anxiety). Taper down symmetrically at cycle end to avoid rebound hypothyroidism.
Start here. Every protocol below is ordered lowest-first. Begin at the smallest effective dose, hold about a week to assess tolerance, and step up only if needed.
How it works
Liothyronine is identical to endogenous T3 and acts via nuclear thyroid hormone receptors (TR-alpha and TR-beta). Binding recruits co-activators and activates transcription of genes that regulate basal metabolic rate, oxygen consumption, protein turnover, carbohydrate and lipid metabolism, and heat production. T3 drives mitochondrial biogenesis and increases Na+/K+-ATPase activity, raising cellular energy expenditure.
Liothyronine is identical to endogenous T3 and acts via nuclear thyroid hormone receptors (TR-alpha and TR-beta). Binding recruits co-activators and activates transcription of genes that regulate basal metabolic rate, oxygen consumption, protein turnover, carbohydrate and lipid metabolism, and heat production. T3 drives mitochondrial biogenesis and increases Na+/K+-ATPase activity, raising cellular energy expenditure.
Compared to T4, T3 has roughly 4x the receptor-level potency and does not require peripheral deiodination, so its onset is within hours rather than days. The same property makes overshoot easier - a dose change takes effect quickly, and the user feels the effects (both desired and adverse) sooner.
Exogenous T3 suppresses endogenous thyroid production via negative feedback on the HPT axis. On stopping, there is a lag of several days to weeks before endogenous production resumes normal levels, which is why tapered discontinuation is standard. At bodybuilding doses (50-100 mcg/day), cardiac workload rises meaningfully and muscle protein catabolism can offset the desired lean-mass preservation if insufficient caloric and protein intake is provided.
What to expect
First 24-72 hours: warming sensation, mild tachycardia, slight appetite reduction. Effects noticeable within hours of first dose, unlike T4 which takes days.
Week 1-3: measurable fat loss under caloric deficit, elevated basal body temperature, reduced cold tolerance. Cardiac load is palpable - monitor resting heart rate daily.
Week 3-5: peak metabolic acceleration. Extending beyond 5-6 weeks of supraphysiologic dosing without a break compounds cardiac and catabolic risk. Taper down symmetrically to avoid rebound hypothyroid phase.
Evidence
Interactions & stacking
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Side effects & safety
Commonly reported
- Elevated resting heart rate / tachycardia
- Heat intolerance, increased sweating
- Mild tremor, anxiety, jitteriness
- Sleep disturbance (especially if dosed late in the day)
- Increased appetite or transient nausea
- Muscle protein catabolism at higher doses (potential strength loss if diet is under-supportive)
Less common & notes
- Palpitations, atrial fibrillation (especially in older users or those with underlying cardiac disease), chest pain, hypertension, accelerated bone turnover (osteoporosis risk with chronic supraphysiologic use), menstrual irregularities, hair thinning or hair texture changes.
- Rare but serious: thyroid storm, myocardial infarction at very high doses.
- Abrupt discontinuation from high doses produces a period of functional hypothyroidism (fatigue, weight gain, cold intolerance) lasting 2-6 weeks until endogenous production recovers.
Pharmacokinetics
Storage. Store at controlled room temperature (20-25°C / 68-77°F). Protect from light and moisture - liothyronine is moisture-sensitive. Keep in original container with desiccant where provided. Shelf life typically 2-3 years from manufacture date. Keep out of reach of children - small doses have outsized effect in children.
Regulatory status
FDA-approved under brand names Cytomel (Pfizer / King Pharmaceuticals) and Triostat (IV formulation). Prescription-only in the United States. Not a controlled substance. Not prohibited by WADA - thyroid hormones are not on the current WADA prohibited list, though they are monitored and some sporting bodies impose therapeutic use exemption requirements. Internationally available under various brand names including Thybon, Cynomel, and Tertroxin.
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