Thyroideahormoner

Thyroideahormoner are the hormones produced by the thyroid gland, principally 3,5,3'-triiodothyronine (T3) and thyroxine (T4). The majority of secreted hormone is T4, which serves largely as a prohormone, while T3 is the more active form. Most T3 is formed by deiodination of T4 in peripheral tissues. Hormone synthesis occurs in thyroid follicular cells, where iodine is taken up and incorporated into tyrosine residues of thyroglobulin to form precursors that are coupled to produce T4 and T3, which are then released into the bloodstream after proteolysis of thyroglobulin.

Regulation of thyroid hormone production is governed by the hypothalamic–pituitary–thyroid axis. The hypothalamus releases thyrotropin-releasing hormone (TRH), stimulating the pituitary to release thyroid-stimulating hormone (TSH). TSH promotes the synthesis and release of T4 and T3. Circulating free T4 and T3 exert negative feedback on the hypothalamus and pituitary, modulating TRH and TSH secretion to maintain homeostasis.

In the blood, most thyroid hormones are bound to transport proteins, mainly thyroxine-binding globulin (TBG), albumin, and transthyretin. Only the unbound (free) fractions of T4 and T3 are biologically active. Thyroid hormones act broadly to regulate metabolic rate, energy expenditure, and heat production; they are essential for normal growth and development, particularly of the brain and nervous system, and influence cardiovascular function and gut motility.

Clinical aspects include hypothyroidism (underproduction) and hyperthyroidism (overproduction). Diagnosis typically relies on serum TSH with free T4 (and sometimes free T3). Treatments include levothyroxine for hypothyroidism and antithyroid drugs, radioactive iodine, or surgery for hyperthyroidism, with ongoing monitoring of TSH and free hormone levels. Pregnancy and neonatal development require careful management of thyroid hormone levels.

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