IODINE METABOLISM AND THE THYROID - The thyroid gland concentrates iodide which is oxidised in the gland and later released as iodinated amino acids thyroxine (tetra-iodothyronine) (T4) and liothyronine (tri-iodothyronine) (T3) - mainly T4. T4 and T3 are transported in the blood almost completely bound to thyroxine binding globulin (TBG). Thyroid hormone release is regulated by thyroid stimulating hormone (TSH) from the pituitary, which is in turn controlled by thyrotrophin releasing factor from the hypothalamus. This mechanism responds to plasma levels of thyroid hormone. In hypothyroidism due to disease of the thyroid itself (primary hypothyroidism), blood levels of TSH are high; in hypothyroidism due to pituitary disease (secondary hypothyroidism) they are low. Iodide is largely excreted passively through the kidney. Extra-thyroidal sites of iodide concentration are not influenced by TSH, but may be relevant to the diagnostic and therapeutic use of radio-iodine, e.g. during lactation.
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