thyreoideahormon

Thyroid hormone, also known as thyreoideahormon in some languages, refers to the hormones produced by the thyroid gland, primarily thyroxine (T4) and triiodothyronine (T3). Iodine is essential for their synthesis. T3 is the more biologically active form, while T4 is produced in larger amounts and serves largely as a precursor that can be converted into T3 in peripheral tissues.

Production and regulation occur in the thyroid follicular cells. Iodide is taken up by the sodium-iodide symporter, then organified and coupled by thyroid peroxidase to form the hormones, which are stored bound to thyroglobulin in the colloid. They are released into the bloodstream by endocytosis and proteolytic processing. Most circulating T4 is secreted as T4; T3 is produced directly and also generated by deiodination of T4 in tissues via deiodinase enzymes. The hypothalamus-pituitary-thyroid axis tightly regulates production through thyrotropin-releasing hormone (TRH) and thyroid-stimulating hormone (TSH), with negative feedback from circulating T4 and T3.

Circulating thyroid hormones are mostly bound to transport proteins such as thyroxine-binding globulin and albumin; only the free fraction is biologically active. They act by binding nuclear thyroid hormone receptors (TRα and TRβ) to regulate gene expression, influencing basal metabolic rate, heat production, protein synthesis, carbohydrate and lipid metabolism, and cardiovascular function. They are essential for growth and development, especially brain maturation in the fetus and early life.

Clinical relevance includes hypothyroidism and hyperthyroidism, goiter, and iodine deficiency disorders. Treatment may involve thyroid hormone replacement (commonly synthetic T4) or therapies that reduce hormone production in hyperthyroidism.