Final answer:
Thyroxine (T4) is an amino acid-derived hormone produced by the thyroid gland that regulates metabolism and gene expression. It is synthesized from tyrosine and iodine and is most commonly converted into the more potent T3 in target tissues. Malfunction in T4 production can lead to various thyroid disorders.
Step-by-step explanation:
Thyroxine, or tetraiodothyronine (T4), belongs to the group of amino acid-derived hormones and is secreted by the thyroid gland. It is synthesized from the amino acid tyrosine in the presence of iodine. The thyroid gland takes up iodine ions from the plasma through an active transport mechanism, which relies on the Na+, K+-ATPase pump and energy consumption.
Once inside the gland, enzymes convert iodide to its active form for incorporation into T4 and T3 (triiodothyronine) with the help of thyroglobulin, a glycoprotein found in the thyroid. T4, which contains four iodine atoms, is the more abundant form of thyroid hormone, comprising about 90% of circulating thyroid hormones, while T3 has three iodines and is the more potent form. Most of T4 is converted to T3 by target tissues. These thyroid hormones play a crucial role in regulating the basal metabolic rate and gene expression by crossing cell membranes, binding to intracellular receptors, and increasing metabolism.
Thyroid hormone production is under the control of thyroid-stimulating hormone (TSH) from the anterior pituitary, which, in turn, is regulated by thyrotropin-releasing hormone (TRH) from the hypothalamus. Abnormal levels of these hormones can lead to conditions such as hyperthyroidism or hypothyroidism, which can have significant health impacts.