Peripheral Transport and Metabolism of Thyroid Hormones

More than 99% of circulating thyroid hormones are bound to serum proteins, including thyroxine-binding globulin (TBG), transthyretin, and albumin.27 TBG is a glycoprotein that contains only one binding site per molecule. TBG is responsible for the transport of more than three fourths of thyroid hormone in the blood, and its levels are significantly increased by elevated levels of estrogens, as occurs in pregnancy. Dissociation of the free hormone from its binding proteins is rapid and efficient. Thyroid hormones are lipophilic and are capable of passive diffusion into cells, although specific transporters may also regulate intracellular thyroid hormone content.28

T3 synthesized directly by the thyroid forms a relatively small proportion of the effective T3 concentration in tissues, which is mainly derived from peripheral deiodination of T4. This reaction is catalyzed by two deiodinases with characteristic tissue distributions. Type I deiodinase (5'DI) is predominant in liver, kidney, and thyroid, whereas type II deiodinase (5'DII) is present in the central nervous system, pituitary, placenta, brown adipose tissue, cardiac and skeletal muscle, and thyroid.27 A third deiodinase (5'DIII) catalyzes deiodination of T4 to rT3 orT3 to diiodothyronine (T2) and is found in the placenta and central nervous system.27 These differences in distribution and regulation may explain some tissue-specific variation in thyroid hormone action. Peripheral conversion of T4 to T3 may be impaired in a number of situations, including systemic illness, malnutrition, and trauma or by various drugs.

The thyroid hormones generally have slow turnover times in the peripheral circulation. In adults, the half-life of T4 is about 7 days, presumably because of the high degree of binding of T4 to its carrier proteins, whereas the half-life of T3 is approximately 8 to 12 hours.

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