Thyroxine

Thyroxine, or T4, is an iodine-containing hormone produced by the thyroid gland and the major thyroid hormone secreted into the circulation. It is also called tetraiodothyronine because each molecule contains four iodine atoms. T4 is synthesized within thyroid follicles from iodinated tyrosine residues in the protein thyroglobulin and is released when thyroglobulin is broken down by thyroid cells .¹

Most circulating T4 is bound to transport proteins, while only a small fraction circulates in the unbound, or free, form. T4 serves importantly as a circulating precursor for triiodothyronine (T3). Enzymes called deiodinases can remove an iodine atom from T4 in tissues to produce T3, allowing individual tissues to regulate local thyroid hormone activity .²

T3 binds thyroid hormone receptors with substantially greater affinity than T4 and mediates many of the well-established genomic effects of thyroid hormone. These receptors regulate gene expression involved in processes such as metabolism, growth, development, and tissue function. T4 is therefore commonly described as a prohormone, although it can also have biological actions of its own .^2,3

T4 should not be confused with T3 or thyroid-stimulating hormone (TSH). T3 is a more biologically potent thyroid hormone formed both in the thyroid and from T4 in other tissues. TSH, by contrast, is a pituitary hormone that stimulates thyroid hormone synthesis and secretion .^1,2