Fig. 1.

Schematic diagram of thyroid and its biological function. A. Thyrotropin releasing hormone (TRH) is secreted by the hypothalamus and acts on the pituitary gland causing the release of TSH. TSH acts on the thyroid gland, stimulating the release of thyroid hormone. TRH and TSH are synergistically regulated and the thyroid and the pituitary gland are working under a feedback loop. B. Detailed presentation of a thyroid cell. C. Specific illustration of iodine uptake by thyroid follicular cells. Thyroid epithelial cells take up iodine via sodium-iodine symporters. It is secreted into the follicle through the chloride or iodine transporter pendrin on the apical side. Thyroglobulin secretion is processed based on the uptake of amino acids on the basolateral side. Thyroglobulin within thyroid follicles is then iodinated and subsequently taken up by follicular cells via endocytosis. After treatment with proteases, thyroid hormones are released into the blood.