European Journal of Biomedical and Pharmaceutical Sciences

THYROID GLAND

Priyangaa Sathasivam* and Dr. Naveen Kumar M.

ABSTRACT

The thyroid gland secretes thyroxine (T4) and triiodothyronine(T3), both of which modulate energy utilization and heat production and facilitate growth.The gland consists of two lateral lobes joined by an isthmus. The weight of the adult gland is 10 to 20 g.Microscopically, the thyroid is composed of several follicles containing colloid surrounded by a single layer of thyroid epithelium. The follicular cells synthesize thyroglobulin,which is then stored as colloid. Biosynthesis of T4 and T3 occurs by iodination of tyrosine molecules in thyroglobulin. Dietary iodine is essential for synthesis of thyroid hormones. Iodine, after conversion to iodide in the stomach, is rapidly absorbed from the gastrointestinal tract and distributed in the extracellular fluids. After active transport from the blood stream across the follicular cell basement membrane, iodide is enzymatically oxidized by thyroid peroxidase, which also mediates the iodination of the tyrosine residues in thyroglobulin to form monoiodotyrosine and diiodotyrosine. The iodotyrosine molecules couple to form T4 (3,5,3￠,5￠- tetraiodothyronine) or T3 (3,5,3￠-triiodothyronine). Once iodinated, thyroglobulin containing newly formed T4 and T3 is stored in the follicles. Secretion of free T4 and T3 into the circulation occurs after proteolytic digestion of thyroglobulin, which is stimulated by thyroid-stimulating hormone (TSH). Deiodination of monoiodotyrosine and diiodotyrosine by iodotyrosine deiodinase releases iodine, which then reenters the thyroid iodine pool.

Keywords: Thyroid Gland, Iodine, Thyroid.