The thyroid, located at the base of the neck, produces thyroxine (T4) and triiodothyronine (T3), two important hormones containing iodine. T4 and T3 stimulate skeletal growth, nerve tissue and blood cells, promote nutrient metabolism and heat production, and much more. Their production is controlled by TSH, a hormone produced by the pituitary gland, located at the base of the brain. The secretion of TSH by the pituitary gland is modulated by factors related to the body’s needs, especially the levels of T4 and T3 circulating in the blood. When T4 and T3 levels are too low to meet the body’s needs (hypothyroidism), the pituitary gland reacts by secreting more TSH to overstimulate the thyroid. As soon as T4 and T3 levels are restored, pituitary gland secretion returns to normal. Conversely, if T4 and T3 levels are too high (hyperthyroidism, overmedication), the pituitary gland completely stops producing TSH.
The thyroid profile includes a test for TSH, T4 and T3, sometimes performed all at once (complete), sometimes in a cascade (TSH first, then T4 and T3 only when the TSH result is abnormal). It detects most abnormalities affecting the thyroid or the secretion of TSH by the pituitary gland. (Refer to the sections concerning TSH, free T4, T3, thyroglobulin, antithyroid antibodies and antithyroglobulin antibodies.)