The Thyroid Gland

The Thyroid Gland

The thyroid gland (Figure 6.1 and 6.2), located immediately below the larynx on each side of and anterior to the trachea, is one of the largest endocrine glands, normally weighing 15 to 20 grams in adults. The thyroid secretes and store two major hormones, thyroxin and triiodothyronine, commonly called T4 and T3, respectively (Table 6.1). Both of these hormones profoundly increase the metabolic rate of the body; regulate the heart rate, blood pressure, body temperature and facilitate growth. Complete lack of thyroid secretion usually causes the basal metabolic rate to fall 40 to 50 per cent below normal, and extreme excesses of thyroid secretion can increase the basal metabolic rate to 60 to 100 per cent above normal. Thyroid secretion is controlled primarily by thyroid-stimulating hormone (TSH) secreted by the anterior pituitary gland. The thyroid gland also secretes calcitonin, an important hormone for calcium metabolism. Iodine is required for the formation of thyroxin. To form normal quantities of thyroxin, about 50 milligrams of ingested iodine in the form of iodides are required each year, or about 1 mg/week.

Table 6.1 Thyroid Function Test