trijódtironin

Trijódtironin, known in English as triiodothyronine and abbreviated T3, is a thyroid hormone containing three iodine atoms. It is produced in the thyroid gland from the precursor thyroglobulin and is also generated in peripheral tissues by the enzymatic deiodination of thyroxine (T4). T3 is the more active hormone of the two main thyroid hormones, with higher affinity for thyroid hormone receptors than T4.

In physiological terms, T3 regulates cellular metabolism, growth, development, and thermogenesis. It exerts its effects by binding to nuclear thyroid hormone receptors (primarily TR alpha and TR beta), which modulate the expression of numerous genes involved in energy use, protein synthesis, and mitochondrial activity. Although the thyroid gland secretes some T3 directly, most circulating T3 is produced by peripheral conversion of T4, mainly in the liver and kidney, through deiodinase enzymes. T3 circulates in the blood largely bound to transport proteins, with only a small fraction existing as free (biologically active) hormone.

Clinically, synthetic liothyronine (synthetic T3) is used in certain thyroid disorders and in some short-term treatment regimens for hypothyroidism, though T4 monotherapy remains more common. T3 therapy requires careful monitoring due to its potent and shorter-acting nature, and it can cause symptoms of thyrotoxicosis such as palpitations, heat intolerance, weight loss, and anxiety, especially in patients with heart disease or older individuals.

Laboratory assessment of thyroid status often includes free T3 measurements alongside TSH and free T4 to provide a fuller picture of thyroid function.