Thyroid Function Tests (TFT)
The thyroid gland develops in the area of the tongue and then descends anterior to the pharynx to reach its normal position in the neck in front of the tracheal cartilage. Aberrant thyroid tissue may occur along this path of descent.
The thyroid gland consists of follicles lined by cells which secrete thyroglobulin (also known as the colloid). Iodide from the diet is used as iodine to iodinate the tyrosine amino acid residues of the thyroglobulin protein to give monoiodotyrosine (MIT) and diiodotyrosine (DIT). These couple -
MIT + DIT = T3
DIT + DIT = T4 = thyroxine
The thyroglobulin with the T4 and T3 attached is taken up from the colloid into the follicular cells where proteases release T3 and T4. More T4 than T3 is produced from the thyroid gland, but T4 is converted to the more active T3 in some peripheral tissues.
In plasma most of the T4 and T3 exists bound to thyroxine binding globulin (TBG), thyroxine binding prealbumin and albumin. Only free hormone, free T4 (fT4) and free T3 (fT3) is available for tissue action.
MIT + DIT = T3
DIT + DIT = T4 = thyroxine
The thyroglobulin with the T4 and T3 attached is taken up from the colloid into the follicular cells where proteases release T3 and T4. More T4 than T3 is produced from the thyroid gland, but T4 is converted to the more active T3 in some peripheral tissues.
In plasma most of the T4 and T3 exists bound to thyroxine binding globulin (TBG), thyroxine binding prealbumin and albumin. Only free hormone, free T4 (fT4) and free T3 (fT3) is available for tissue action.
The control over the amount of thyroid hormones (T4 and T3) in the plasma is achieved via the release of hormones from the hypothalamus (thyrotrophin releasing hormone - TRH) and the anterior pituitary (thyroid stimulating hormone - TSH).
Click on the thyroid gland to continue
