Last updated: March 2026
The metabolically active form of thyroid hormone. 4-5x more potent than T4. Short half-life requires divided dosing. The T4+T3 combination therapy debate continues.
T3 (liothyronine) is the body's active thyroid hormone. While T4 (levothyroxine) is the major secretion from the thyroid, T3 is the form that actually binds to thyroid receptors and drives metabolic rate.
T3 binds directly to thyroid hormone receptors (TRα and TRβ) with high affinity. This direct activation is why T3 is 4-5x more potent than T4 on a molar basis.
The body converts T4 to T3 via deiodinase enzymes (mainly type 1 and type 2). Some patients have genetic variations affecting this conversion, leading to the "T3 trial" concept.
T3 has a half-life of approximately 1 day (vs T4's 7 days). This requires divided dosing (usually 2x/day) to maintain stable levels. Also causes faster reversal if discontinued.
Some patients and physicians advocate adding T3 to standard T4 therapy. This is controversial — studies show mixed results. Patient preference plays a significant role in treatment decisions.
Context: Most hypothyroidism patients do well on T4 monotherapy. The subset who don't respond fully may have conversion issues or T3 receptor polymorphisms. Combination therapy remains controversial with mixed study results.
Support for thyroid health.
Dosing schedules, interaction warnings, and cycle protocols for 50+ compounds — all in one place.
Thyroid medication requires a prescription and medical supervision. T3 (liothyronine) is FDA-approved but must be monitored with blood tests (TSH, Free T3, Free T4). Never adjust thyroid medication without consulting your healthcare provider. This is not medical advice. FDA Approved Rx Required