Thyroid hormone

Thyroid hormone binds to specific thyroid hormone receptors and modify gene transcription in almost all tissues.

Iodine is a substrate for thyroid hormone synthesis and is actively transported into the thyroid fully to the cells by an iodine symporter.

Iodine is oxidized and bound covalently to the tyrosyl residues of thyroglobulin.

Iodine is essential for free T4 synthesis.

The coupling of iodinate tyrosyl residues results in the formation of T4 and T3, which are stored within the colloid space.

Thyroid hormone also has nongenomic activities.

Increases the expression of beta-adrenergic receptors and cyclic AMP isoforms.

Reduces expression of inhibitory G-protein subunits and contributes to thermogenesis (Silva JE, Blanco SDC).

It modulates hepatic glucose and lipid metabolism.

Older women with excess thyroid hormone have an increased risk for hip and vertebral fractures.

Thyroid hormone lowers the level of serum LDL cholesterol.

Thyroid hormone analogue eprotirone utilized in a randomized, placebo controlled, double blind trial in patients with hypercholesterolemia on statins was associated with decreases in serum LDL levels and similar reductions in serm apoliprotein B, triglycerides and Lp(a) lipoprotein (Ladenson PW).

Active secretion of thyroid in the fetus starts at 18-20 weeks gestation.

In iodine deficient population, iodine supplementation increases cognitive performance if given before pregnancy.

In the first trimester the fetus is dependent on maternal circulating free T4 synthesis for growth and development of CNS maturation.

A low serum thyrotropin level is the best test to detect thyroid dysfunction, and the highest sensitivity and specificity for the diagnosis of thyroid dysfunction.

Thyroid hormone may circulate is T3, or is T4, a prohormone, which is converted to T3 in peripheral tissues.

T3 is the physiologically active form of thyroid hormone.

Free T4 levels can be used to assess the degree of hyperthyroidism.

T3 levels can also help establish the cause and severity of hyperthyroidism.

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