Thyroid Hormones

Thyroid hormones are iodine-containing hormones that regulate metabolism, growth, development, and the function of many tissues. The two principal thyroid hormones are thyroxine (T4) and triiodothyronine (T3). They are produced from iodinated tyrosine residues within the protein thyroglobulin in thyroid follicles and are released into the bloodstream by the thyroid gland .¹

The thyroid secretes predominantly T4 and a smaller amount of T3. T4 serves importantly as a circulating precursor for T3. In tissues, enzymes called deiodinases can remove an iodine atom from T4 to produce T3. Other deiodination reactions can instead produce inactive metabolites, allowing individual tissues to regulate local thyroid hormone activity partly independently of circulating hormone concentrations .²

T3 is the more biologically potent thyroid hormone and acts mainly through thyroid hormone receptors. These nuclear receptors regulate gene transcription when bound by thyroid hormone, producing effects that vary among tissues. Through these mechanisms, thyroid hormones influence processes including energy metabolism, development, and cellular differentiation .^2,3

T4 and T3 should be distinguished from thyroid-stimulating hormone (TSH). TSH is not a thyroid hormone; it is produced by the pituitary gland and stimulates the thyroid, regulating many of the steps involved in thyroid hormone synthesis and release .¹