ABSTRACT

The thyroid gland is a highly vascular structure made up of two lobes, joined together by the thyroid isthmus. The isthmus passes in front of the trachea overlying the second to fourth tracheal rings in the adult. The epithelial cells make thyroid hormones and secrete them into the cavity, where they are stored bound to thyroglobulin, which is a globular colloidal substance. There are three different thyroid hormones: thyroxine (T4), 3,5,3-triiodothyronine (T3) and 3,3,5-triiodothyronine (reverse T3). The majority of hormone synthesised is T4, but the active hormone is actually T3, which is five times more potent. Thyroid-stimulating hormone (TSH) stimulates the release of the hormones by driving the endothelial cells to take in the colloidal thyroglobulin by pinocytosis. The hypothalamus produces thyroid-releasing hormone, which stimulates the release of TSH from the anterior pituitary. Thyroid hormones exert a direct effect and have a permissive effect on growth hormone.