T3 Cytomel
About
About T3 Cytomel
Liothyronine (T3) is the biologically active thyroid hormone that directly activates thyroid hormone receptors in cells, in contrast to T4 which requires conversion. It is FDA-approved for hypothyroidism and is used off-label in optimization medicine to address tissue hypothyroidism, low T3 syndrome, and refractory depression. Its short half-life requires multiple daily doses but provides more direct cellular thyroid activity than T4 alone.
Science
Mechanism of Action
Directly binds thyroid hormone receptors (TRα and TRβ) in cell nuclei to regulate gene expression controlling metabolic rate, cardiac function, thermogenesis, and neurological activity without requiring peripheral deiodination conversion from T4.
Dosing
Typical Protocol
5–50 mcg oral 1–3x daily; often combined with T4 in 4:1 T4:T3 physiologic ratio for combination thyroid therapy.
⚠ Protocol information is for educational purposes only. Dosing must be determined by a licensed physician based on individual health status and goals.
Regulatory
Legal Status in 2026
This compound has completed clinical trials and received formal FDA approval for one or more indications. It can be legally prescribed by licensed physicians, filled at any licensed pharmacy, and may be covered by insurance. Off-label use by physicians is also legal. This is the highest level of regulatory clearance available in the US.
Evidence
Evidence Tier
Supported by the clinical trial data required for FDA approval, representing the highest evidence bar. Safety and efficacy have been established in multiple phases of human clinical trials.
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