Thyroid hormones synthesis regulation

2. SYNTHESIS OF THYROID HORMONES

      The metabolism of iodine leading to the biosynthesis of thyroid hormones occurs in sequential stages:

*     The first step is active transport of iodine into the thyroid (iodine trap)

*       Oxidation of iodide and iodination of tyrosil residues within thyroglobulin to yield the hormonally inactive iodotyrosines mono and diodotyrosines (MIT and DIT) by thyroid peroxidase enzyme.

*     The coupling of these MIT and DIT to form hormonally active iodothyroines, notably T4and T3.

3.REGULATION OF HYPOTHALAMIC-

PITUITARY THYROID COMPLEX

      TSH is a glycoprotein hormone secreted by a specific cell type, the thyrotropic cell, located principally in the anteromedial portion of the anti pituitary.

*     TSH is the major regulator of the morphological and functional state of the thyroid.

*     Removal of TSH stimulation is followed by hypovascularity and atrophy of the thyroid gland accompanied by decreased sythesis and secretion of hormone (T4, T3)

*     The effects are reversed by stimulatory doses of TSH

*     Regulation of TSH secretion results from a complex interaction, mainly or entirely at the level of the pituitary thyrotropic cell.

*     By which Thyrotrophin releasing hormone (TRH) produced in the hypothalamus acts to stimulate the release and later the synthesis of TSH.

*     While high or low thyroid hormone levels (T4, T3) inhibit and stimulate these functions through Negative and positive feed back action 

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