T4 Levothyroxine
About
About T4 Levothyroxine
Levothyroxine is the synthetic form of T4 and the most prescribed thyroid medication in the US, used as first-line treatment for hypothyroidism. It is a prodrug converted peripherally to active T3 by deiodinase enzymes, making conversion efficiency a critical variable in individual response. Many optimization-oriented physicians supplement T4 therapy with T3 to address conversion impairment and residual hypothyroid symptoms.
Science
Mechanism of Action
Serves as a prodrug that is deiodinated in peripheral tissues (primarily liver and kidney) to active triiodothyronine (T3) by type I and II deiodinase enzymes, which then binds thyroid hormone receptors to regulate metabolism and cellular energy production.
Dosing
Typical Protocol
25–200 mcg oral once daily on an empty stomach; titrated based on TSH, free T3, and free T4 levels every 6–8 weeks.
⚠ 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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