Hypothyroidism
Delbert A. Fisher MD1
1 President, Nichols Academic Associates, and Chief Science Officer, Nichols Institute, San Juan Capistrano, CA.
Thyroid Development
THYROID HORMONE SYNTHESIS AND CONTROL
Thyroid hormones (TH) are critical for normal growth, development, and metabolism during infancy and childhood. They are synthesized by the thyroid gland follicular cells, the function of which is to concentrate iodide from the blood and return it to peripheral tissues in a hormonally active form. The thyroid cell iodide-concentrating mechanism, often referred to as the iodide pump, confers on the gland its ability to concentrate iodide to many times its level in plasma. The steps involved in synthesis and release of TH include: 1) iodide trapping by the thyroid gland; 2) synthesis of thyroglobulin; 3) organification of trapped iodide as iodotyrosines (monoiodotyrosine, MIT, and diiodotyrosine, DIT); 4) coupling of the iodotyrosines to form the iodothyronines thyroxine (T4) and triiodothyronine (T3) and storage in follicular colloid; 5) endocytosis of colloid droplets and hydrolysis of thyroglobulin to release MIT, DIT, T4, and T3; and 6) deiodination of MIT and DIT with intrathyroidal recycling of the iodine.
The concentration of iodide in plasma under normal circumstances is less than 0.2 µg/dL. The iodide is removed from plasma almost entirely by the kidney and the thyroid gland. Transport of iodide across the thyroid cell membrane is the first and ratelimiting step in TH biosynthesis.
