Triiodothyronine

Triiodothyronine, or T3, is an iodine-containing thyroid hormone and the more biologically potent of the two principal thyroid hormones in humans. Its molecule contains three iodine atoms, which gives rise to the name triiodothyronine. The thyroid gland produces some T3 directly but secretes substantially more thyroxine, or T4 .^1,2

Most circulating T3 is produced outside the thyroid by conversion of T4 in peripheral tissues. Enzymes called iodothyronine deiodinases remove one iodine atom from T4 to generate T3. In healthy people with adequate iodine intake, approximately 80% of daily T3 production arises through this conversion rather than direct thyroid secretion. Other deiodination pathways can instead convert thyroid hormones into less active or inactive metabolites .¹

T3 exerts many of its effects by binding to thyroid hormone receptors, primarily TRα and TRβ. These nuclear receptors regulate gene transcription in response to T3. Through tissue-specific receptor signaling, T3 contributes to the regulation of energy metabolism, growth, development, and the function of numerous organs .²

T3 is closely related to, but distinct from, T4. T4 contains four iodine atoms and functions largely as a circulating precursor from which T3 can be generated. T3 binds thyroid hormone receptors with greater affinity and is more biologically active than T4. T3 should also not be confused with reverse T3, an alternative three-iodine metabolite of T4 with little thyroid hormone receptor activity .^1,2