T3 (Triiodothyronine)
Liothyronine · Cytomel · T3
Liothyronine (T3); short half-life requiring divided dosing.
Key takeaways
- Synthetic T3 (Cytomel), approved for hypothyroidism; fat-loss use is off-label and risky.
- ~22-hour half-life, ~95% oral absorption; split daily dosing is the common reported pattern.
- Suppresses TSH via pituitary feedback, and recovery can lag weeks after stopping.
- Excess T3 costs muscle and bone and strains the heart, not just fat.
Overview
What it is
Liothyronine (Cytomel) is synthetic T3, the active thyroid hormone, approved for hypothyroidism. It also sees off-label use for fat loss, where short-term metabolic rate is bought with real endocrine and cardiac risk.
Pharmacokinetics
T3 absorbs almost completely (~95%) with a roughly 22-hour half-life and a peak within a few hours, which is why split daily dosing is the common reported pattern. Exogenous T3 suppresses TSH through pituitary feedback, and direct T3 dosing produces peaks that normal thyroid secretion never generates.
What the evidence says
As replacement therapy T3 is well characterized clinically, though its approval predates modern trial standards and levothyroxine remains the guideline default. At the supraphysiological doses seen in fat-loss use, the documented costs are tachycardia, arrhythmia risk, muscle catabolism, and bone loss, with TSH suppression that can persist for weeks after stopping.
Reported dosing protocols
Protocols reported in research literature and clinic/community use — informational context, not a dosing recommendation.
| Use case | Dose | Route | Frequency | Duration |
|---|---|---|---|---|
| Hypothyroidism (label) | 25 mcg start, titrate by labs | Oral | Once daily | Chronic |
| Fat loss (community) | 25–75 mcg | Oral | 1–3× daily | 4–8 weeks |
Pharmacokinetic summary
- Model tier
- Advanced (t½ + Cmax + Tmax + F)
- Half-life
- 22.0 hours
- Cmax
- 6.92 ng/dL
- Tmax
- 0.1 days
- Bioavailability
- 95%
- Typical dose
- 0.025–0.075 mg
Frequently asked questions
T3 vs T4 (levothyroxine) — why the debate?
Levothyroxine converts to T3 in the body and gives smooth levels; liothyronine delivers T3 directly with sharp peaks. Guidelines still favor T4 alone for hypothyroidism, reserving combination therapy for selected cases.
Does T3 shut down your thyroid?
Exogenous T3 suppresses TSH and reduces endogenous output while you take it. Function typically recovers after discontinuation, but the axis can take several weeks to normalize, which is why tapering is common practice.
Is T3 effective for fat loss?
It raises metabolic rate, but the calorie burn comes with proportional costs: tachycardia, arrhythmia risk, muscle wasting, and bone loss at supraphysiological doses. Lost weight includes lean tissue, and regain after stopping is common.
References
- PMC T3 PK
- Single-dose T3 kinetics (Ther Drug Monit 2015) — Entry's PK source; single-dose pharmacokinetics and physiological effects.
- DailyMed — Cytomel prescribing information — Label dosing and titration for hypothyroidism.
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