A highly vascular organ situated in front of the neck. It consists of a narrow isthmus crossing the windpipe close to its upper end, and joining two lateral lobes which run upwards, one on each side of the LARYNX. The gland is therefore shaped somewhat like a horseshoe, each lateral lobe being about 5 cm (2 inches) long and the isthmus about 12 mm (½ inch) wide, and it is firmly bound to the larynx. The weight of the thyroid gland is about 28.5 grams (1 ounce), but it is larger in females than in males, and in some women increases in size during MENSTRUATION. It often reaches an enormous size in the condition known as GOITRE (see also THYROID GLAND, DISEASES OF).
The chief function of the thyroid gland is to produce a hormone (HORMONES) rich in iodine – THYROXINE, which controls the rate of body METABOLISM. Congenital deficiency results in short stature and LEARNING DISABILITY, a condition formerly known as CRETINISM. If the deficiency develops in adult life, the individual becomes obese, lethargic, and develops a coarse skin, a condition known as hypothyroidism (see under THYROID GLAND, DISEASES OF). Overactivity of the thyroid, or hyperthyroidism, results in loss of weight, rapid heart action, anxiety and increased appetite. (See THYROID GLAND, DISEASES OF – Thyrotoxicosis.)
The production of the thyroid hormone is controlled by a hormone of the PITUITARY GLAND – the thyrotrophic hormone.