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Thyroid Gland and Parathyroid
1. The thyroid gland The thyroid gland lies on the trachea directly behind the larynx, close to the recurrent laryngeal nerve, carotid sheath and sternohyoid and sternothyroid muscles. The parathyroid glands are located dorsally to or within the thyroid gland itself. Its form varies between species. In the dog and cat it consists of separate masses that are occasionally connected by an isthmus. In the horse, paired lobes are present that are connected by an insubstantial isthmus only. In cattle, the lobes are connected by a wide isthmus of parenchymal tissue. In pigs, the thyroid is more compact and has a large median pyramidal lobe as well as lateral lobes. It is supplied by the cranial thyroid artery, a branch of the common carotid artery. A subsidiary supply is provided by the caudal thyroid artery. The cranial and caudal thyroid arteries are united by substantial anastomoses along their caudal edge. Venous drainage is provided by the internal jugular vein. Lymph drains into the cranial deep cervical nodes. The mature thyroid gland is enclosed in a connective tissue capsule loosely attached to neighbouring organs. It is composed of many follicles, so has a granular appearance, and is red in colour. The follicles are bounded by a single layer of cuboidal epithelial (follicular) cells and a basement membrane, surrounding a central lumen filled with a homogenous protein rich colloid called thyroglobulin. The follicular cells are connected by tight junctions, and have a dense capillary network. The apical surface of the cell membrane is covered with numerous microvilli to increase surface area. The colloid is a store of thyroid hormones prior to secretion. In the active gland, colloid is diminished and epithelial cells are tall and columnar. Within the connective tissue close to the follicles are C-cells or parafollicular cells. They are found in clusters in the interfollicular space. They secrete calcitonin, a hormone which lowers plasma calcium levels. There are also clusters of parathyroid cells, which produce parathyroid hormone (PTH), another calcium-regulating hormone. The thyroid gland produces two types of iodine containing hormone, tri-iodothyronine (T3) and thyroxine (T4). The follicular cells synthesise thyroglobulin, which is exocytosed and stored in the colloid. Thyroglobulin is then split to form T3 and T4. T4 contains 4 iodine atoms, whereas T3 contains 3. Most of the hormone produced in the thyroid is T4, as T4 can only be made in the thyroid gland, and can be converted by other tissues to T3. Iodine is actively transported into the follicular cells by an Na+/I- symport in the basal membrane, which can concentrate iodine in the colloid at a level up to 250 times greater than plasma level. This process is known as iodine trapping. The pump is activated by thyroid stimulating hormone (TSH) from the pituitary gland. Both T3 and T4 are formed through the linkage of two iodinated molecules of tyrosine. Iodination of tyrosine molecules linked to thyroglobulin occurs in the apical membrane of the epithelial cells, and is catalysed by the enzyme thyroperoxidae (TPO). Each thyroglobulin molecules contains 120 tyrosine molecules. Addition of an iodine molecule to tyrosine forms monoiodotyrosine (MIT), whilst addition of two iodine molecules forms diiodotyrosine (DIT). Thyroglobulin is then released into the follicular lumen. The iodinated tyrosine molecules are
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