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14.9: Thyroid Hormones (T3 and T4)

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    Source and Control of Secretion

    Most of the hormone-producing cells of the thyroid gland secrete two related thyroid hormones: thyroxine (T4) and triiodothyronine (T3). Target cells convert some T4 to T3 and then release the T3 back into the blood.

    Thyroid hormone secretion is controlled primarily by a negative feedback mechanism that acts through the hypothalamus and anterior pituitary gland. Low blood levels of thyroid hormone result in increased thyroid production, and vice versa. However, thyroid hormone secretion can be influenced by other factors. For example, a low metabolic rate stimulates thyroid hormone secretion. By contrast, high blood levels of somatostatin, glucocorticoids, and sex steroids (e.g., testosterone, estrogen) inhibit secretion.


    Though T3 is more powerful than T4, both cause a general increase in the metabolic rate by increasing the rates of many chemical reactions in most cells. Only certain cells in the brain, spleen, testes, uterus, and thyroid gland are unaffected by thyroid hormones. The general increase in metabolic rate increases heat production, which assists in maintaining normal body temperature. Regulating the metabolic rate also assures that each organ will grow, repair itself, and perform its functions at a proper rate.

    Age Changes

    As age increases, average thyroid hormone blood levels decline slightly, though they remain in the normal range. In addition, there is a decrease in the T3/T4 ratio. These changes may be compensatory because they prevent the development of excessively high metabolic rates in certain cells (e.g., muscle) as lean body mass declines. Therefore, aging does not alter the ability of thyroid hormones to provide proper regulation of their target tissues.

    There is an age-related increase in damage to the thyroid gland by the immune system, which occurs usually in women. The result is in adequate thyroid hormone production, which affects as many as 10 percent of elderly women. Some diagnostic procedures and therapies that use iodine also damage the thyroid gland. The treatment is thyroid hormone supplementation. Excess thyroid production is rare in the elderly. It can result from Grave's disease, excess iodine intake, or thyroid nodules. These conditions are treated easily by removing part or all of the thyroid gland or reducing iodine intake.

    This page titled 14.9: Thyroid Hormones (T3 and T4) is shared under a CC BY-NC-SA 4.0 license and was authored, remixed, and/or curated by Augustine G. DiGiovanna via source content that was edited to the style and standards of the LibreTexts platform.

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