Because thyroid hormone affects the growth and function of many tissues, a deficiency of this hormone in infancy causes physical and mental retardation as well as other symptoms that together constitute congenital hypothyroidism, formerly called cretinism. In the adult, thyroid deficiency causes myxedema, in which there is weight gain, lethargy, rough, dry skin, and facial swelling. Both of these conditions are easily treated with thyroid hormone. Most U.S. states now require testing of newborns for hypothyroidism. If not diagnosed at birth, hypothyroidism will lead to mental retardation within 6 months.
The most common form of hyperthyroidism is Graves disease, also called diffuse toxic goiter. This is an autoimmune disorder in which antibodies stimulate an increased production of thyroid hormone. There is weight loss, irritability, hand tremor, and rapid heart rate (tachycardia). A most distinctive sign is a bulging of the eyeballs, termed exophthalmos, caused by swelling of the tissues behind the eyes. Treatment for Graves disease may include antithyroid drugs, surgical removal of all or part of the thyroid, or radiation delivered in the form of radioactive iodine.
A common sign in thyroid disease is an enlarged thyroid, or goiter. However, a goiter is not necessarily accompanied by malfunction of the thyroid. A simple or nontoxic goiter is caused by a deficiency of iodine in the diet. With the addition of iodine to salt and other commercial foods, this form of goiter has become a thing of the past.
Thyroid function is commonly tested by measuring radioactive iodine uptake (RAIU) by the gland. Blood levels of total and free thyroxine (T4) and triiodothyronine (T3) are also measured, as are the levels of thyroxine-binding globulin (TBG) and thyroid-stimulating hormone (TSH) from the pituitary. Thyroid scans after the administration of radioactive iodine are also used to study the activity of this gland.
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