Thyroxine

Thyroxine, or T4, is a thyroid hormone with four iodine atoms. It circulates in blood and can be converted in tissues into the more active hormone triiodothyronine (T3).

Thyroid gland: An endocrine gland in the neck that produces hormones regulating metabolism, growth, and development. Follicular cells in this gland synthesize and release T4.
Thyroid hormones: Hormones produced by the thyroid that regulate metabolism, development, and other physiological processes. T4 belongs to this hormone family, alongside T3.
Thyroid function tests: Blood tests that assess thyroid status, commonly by measuring TSH and free or total thyroid hormones. T4 measurements help identify and monitor thyroid dysfunction.
Hypothalamic–pituitary–thyroid axis: A feedback system in which the hypothalamus and pituitary regulate thyroid hormone production. T4 feeds back on this axis, influencing TSH secretion and further hormone release.
Thyroglobulin: A large protein made by thyroid follicular cells that supplies tyrosine residues for thyroid hormone synthesis. T4 is assembled from iodinated tyrosines within this protein.
Triiodothyronine: A thyroid hormone with three iodine atoms that generally has greater biological activity than T4. T3 is the more active product formed when T4 loses an iodine atom.
Free thyroxine: The fraction of circulating T4 that is not bound to plasma proteins. Free T4 estimates the hormone fraction immediately available to enter tissues.
Thyroid-stimulating hormone: A pituitary hormone that stimulates thyroid growth and thyroid hormone synthesis and release. Its concentration responds to circulating thyroid hormone levels, including T4.
Thyroid peroxidase: An enzyme that oxidizes iodide and catalyzes the iodination and coupling reactions used to make thyroid hormones. It catalyzes the reactions that form T4 on thyroglobulin.
Iodine: A chemical element required by the thyroid gland to synthesize thyroid hormones. Four iodine atoms are part of each T4 molecule.

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