There is a classical negative feedback loop between T 4and TSH. TSH secretion is mainly regulated by circulating T 4(which is deiodinated to T 3in the pituitary) and to a lesser extent by circulating T 3. Much of the T 3in the blood is generated by the liver after enzymatic removal of an iodine atom from T 4. A very small unbound ('free') fraction is available for uptake by cells. Both hormones are co-secreted with thyroglobulin and circulate in blood bound to thyroid hormone binding proteins (thyroid binding globulin, transthyretin and albumin). In some hyperthyroid states the ratio of T 3to T 4is higher. The uptake of iodide and the release of T 4and T 3are enhanced by thyroid stimulating hormone (TSH) which is secreted by the pituitary gland.Ībout 90% of thyroid hormone released is T 4and 10% is T 3. Iodide then combines with tyrosines in thyroglobulin, mediated by thyroperoxidase, to form T 4(4 iodine atoms) or T 3(3 iodine atoms). The thyroid gland actively transports diet-derived iodide from the blood by means of a cell membrane iodide pump called the sodium-iodide symporter. 1As thyroid disorders may not present with classical clinical signs, it is essential to have accurate assays of thyroid function to assist in the diagnosis. For example, the prevalence of hypothyroidism may be up to nearly 10% of the general population.
These hormones are essential for normal growth, development and metabolic function.Īltered thyroid function is common. The thyroid gland secretes thyroxine (T 4) and triiodothyronine (T 3).
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