The thyroid gland is located in the anterior neck and consists of two lateral lobes joined by a midline isthmus. The weight of the adult gland is around 10-25 grams. The gland is involved with the synthesis, storage and secretion of thyroid hormones.
The functional unit is the follicle or acinus, which is lined with cuboidal epithelium. Cell height varies with the degree of TSH stimulation. The lumen of the follicle is filled with colloid, which consists almost entirely of thyroglobu-lin. The follicles are held together by loose reticular fibres.
Parafollicular or C cells lie between the basement membrane of the follicles and the follicular cells. They are of neural crest origin, being derived from the ultimobranchial bodies of the 4th and 5th branchial pouches. The thyroid gland also possesses a lymphatic network and a network of short, fenestrated capillaries surrounding the follicles.
Ninety per cent of the total body iodine is stored in the thyroid gland, in the form of iodinated amino acids and thyroid hormone. Ten per cent is located in the extracellular pool. Dietary iodine is derived primarily from fish, seafood and seaweed. In goitre endemic regions, iodinated salt or iodinated vegetable oil are employed in prophylaxis.
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