Triiodthyronin

Triiodthyronin, commonly abbreviated T3, is a thyroid hormone that plays a central role in regulating metabolism, growth, and development. In most individuals, the majority of circulating T3 is produced outside the thyroid by deiodination of thyroxine (T4) in peripheral tissues, while the thyroid gland also secretes a smaller amount of T3 directly.

T3 exerts its effects by binding to thyroid hormone receptors (primarily TRα and TRβ) in the nucleus, where it modulates the transcription of numerous target genes. This leads to increased basal metabolic rate, higher energy expenditure, and influences carbohydrate and lipid metabolism, heat production, and protein turnover. Tissue-specific responses to T3 are influenced by local deiodinase activity, which activates or inactivates thyroid hormones within cells.

In the bloodstream, most T3 is bound to transport proteins such as thyroid-binding globulin, albumin, and transthyretin; only a small fraction remains free (free T3), which is the biologically active portion. The half-life of T3 is shorter than that of T4, contributing to more rapid changes in metabolic state.

Regulation of T3 levels is governed by the hypothalamic-pituitary-th thyroid axis. The hypothalamus releases thyrotropin-releasing hormone (TRH), stimulating the pituitary to secrete thyroid-stimulating hormone (TSH), which in turn stimulates the thyroid to produce T3 and T4. Negative feedback by circulating T3 and T4 modulates hypothalamic and pituitary activity.

Clinically, T3 measurements (free T3 or total T3) aid in assessing thyroid function, particularly in suspected hyperthyroidism or certain forms of hypothyroidism.