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).

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.

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