THYROID HORMONES

Thyroid hormones are made by the thyroid gland. Thyroid hormones affect every cell and all the organs of the body.

 They control the rate at which your body burns calories [ your metabolism].

*Can slow down or speed up your heartbeat.

* Can raise or lower your body temperature.

* Change how fast food moves through your digestive tract.

* Affect muscle strength.

* Control how quickly your body replace dying cell.

Thyroid hormones appear to  act directly via T3 receptor but also indirectly by altering expression of genes for neutriophins or their receptors. Too much thyroid hormones speeds things up and too little hormones slow things  down .

 The 1^st step in the synthesis of thyroid hormones is the oxiganification  of lodine  . lodine is taken up  , converted to lodine , and then condensed on to tyrosine residues which reside along the polypeptide backbone of a protein molecule called thyroglobulin. This reaction result in either mono-lodinated tyrosine  [MIT] or di-lodinated tyrosine[DIT] being incorporated into thyroglobulin .This  newly formed lodothyroglobulin forms one of the most important constitunts of the colloid material present in the follicle of the thyroid unit . The other synthetic reaction that is closely linked to organification ,is a coupling reaction where lodotyrosine molecules are coupled together . if two di-lodotyrosine molecules  couple together , the result is the formation of thyroxin [T4].if a di-lodotyrosine and a mono-lodotyrosine are coupled together the result is the formation of tri-lodothyrosine.

From the perspective of the formation of the thyroid hormone, the major coupling to reaction is the di-lodotyrosine coupling to produce T4. Although T3is more biologically active than T4, the  major production of T3 actually occurs outside of the thyroid gland. The majority of T3 is produced by peripherial conversion from T4 in a   diodination reaction involving a specific enzyme which removes one lodine from the outer ring of T4.

The T3 and T4 released from the thyroid by proteclysis reach the blood stream where they are bound to thyroid hormones bindings protein is thyroxin binding globulin [TBG] which accounts for about 75%of the bound homone

In order to attain normal levels of thyroid hormone synthesis , an adequate supply of lodine is essential . the recommended minimum intake of lodine  is 150 micrograms a day . Intake of less than 50 micrograms a day is associated with goiters.

High lodine level inhibit lodine exidation and oxiganification .  Additionally , lodine excess inhibits thyroglobulin proteolysis[this is the principal mechanism for the anti thyroid effect of inorganic lodine in patients with thyrotoxiosis]