How is thyroid hormone synthesized and released?
· Step 1: Iodine, essential for thyroid-hormone synthesis, is reduced to iodide in the stomach and absorbed into the bloodstream by the gastrointestinal tract. Iodide is then absorbed by thyroid follicular cells, which “trap” and concentrate it. Inhibitors of iodide absorption are: circulating iodide, thiocyanate, and perchlorate.
· Step 2: Iodide combines with tyrosine residues on thyro-globulin to form monoiodotyrosine (MIT) and then diiodotyrosine (DIT). Iodinated tyrosines join to create T3 (MIT + DIT) and T4 (DIT + DIT) and bind to thyro-globulin. Inhibitors of T3 and T4 synthesis are: propyl-thiouracil (PTU) and methimazole.
· Step 3: T3 and T4 are cleaved from thyroglobulin and secreted into the circulation. T4 comprises 95% of released hormone. Approximately one third of secreted T4 is converted to T3 in the kidney and liver. Inhibitor of conversion of T4 to T3 is: PTU but not methimazole.