Introduction

The thyroid is a small gland, measuring about 2 inches across, that lies just under the skin below the Adam's apple in the neck. The two halves (lobes) of the gland are connected in the middle (called the isthmus), giving the thyroid gland the shape of a bow tie. Normally, the thyroid gland cannot be seen and can barely be felt, but if it becomes enlarged (goiter), a doctor can feel it easily and a prominent bulge may appear below or to the sides of the Adam's apple.

See the figure Locating the Thyroid Gland.

The thyroid gland secretes thyroid hormones, which control the speed at which the body's chemical functions proceed (metabolic rate). Thyroid hormones influence the metabolic rate in two ways: by stimulating almost every tissue in the body to produce proteins and by increasing the amount of oxygen that cells use. Thyroid hormones affect many vital body functions: the heart rate, the respiratory rate, the rate at which calories are burned, skin maintenance, growth, heat production, fertility, and digestion.

The two thyroid hormones are T₄ (thyroxine) and T₃ (triiodothyronine). T₄, the major hormone produced by the thyroid gland, has only a slight, if any, effect on speeding up the body's metabolic rate. Instead, T₄ is converted into T₃, the more active hormone. The conversion of T₄ to T₃ occurs in the liver and other tissues. Many factors control the conversion of T₄ to T₃, including the body's needs from moment to moment and the presence or absence of other illnesses.

To produce thyroid hormones, the thyroid gland needs iodine, an element contained in food and water. The thyroid gland traps iodine and processes it into thyroid hormones. As thyroid hormones are used, a small amount of the iodine contained in the hormones is released, returns to the thyroid gland, and is recycled to produce more thyroid hormones. Oddly, the thyroid gland releases slightly less of the thyroid hormones if it is exposed to high levels of iodine transported to it in the blood.

The body has a complex mechanism for adjusting the level of thyroid hormones. First, the hypothalamus, located just above the pituitary gland in the brain, secretes thyrotropin-releasing hormone, which causes the pituitary gland to produce thyroid-stimulating hormone (TSH). Just as the name suggests, TSH stimulates the thyroid gland to produce thyroid hormones. The pituitary gland slows or speeds the release of TSH, depending on whether the levels of thyroid hormones circulating in the blood are getting too high or too low.

The thyroid gland also produces the hormone calcitonin, which may contribute to bone strength by helping calcium to be incorporated into bone.

Diagnostic Tests

To determine how well the thyroid gland is functioning, doctors use several tests. Usually the first and best test of thyroid function is measurement of the level of TSH in the blood. Because this hormone stimulates the thyroid gland, blood levels of TSH are high when the thyroid gland is underactive (and thus needs more stimulation) and low when the thyroid gland is overactive (and thus needs less stimulation). However, in rare cases in which the pituitary gland is not functioning normally, the level of TSH does not accurately reflect thyroid gland function.

Doctors can also measure the level of thyroid hormones in the blood. In some cases, the level of a protein called thyroxine-binding globulin is measured as well because it binds the thyroid hormones in the blood. Abnormal levels of this protein can lead to misinterpretation of a person's total thyroid hormone levels but will not affect the levels of unbound or free hormones in the blood, which are the effective forms. The level of thyroxine-binding globulin is lower in people who have kidney disease or who take anabolic steroids. The level is higher in women who are pregnant or taking oral contraceptives or other forms of estrogen and in people in the early stages of hepatitis.

If a doctor feels one or more growths (nodules) in the thyroid gland, he may order a scanning procedure. An ultrasound scan uses sound waves to measure the size of the gland and to determine whether the growth is solid or filled with fluid (cystic). A thyroid scan uses radioactive iodine or technetium and a device to produce a picture of the thyroid gland that will show any physical abnormalities. Thyroid scanning can also help determine whether the functioning of a specific area of the thyroid is normal, overactive, or underactive compared with the rest of the gland.

Additional testing may be necessary in rare cases in which a doctor cannot determine whether the problem lies in the thyroid or in the pituitary gland. One of these tests involves injecting thyrotropin-releasing hormone intravenously and then measuring the level of TSH in the blood to determine the pituitary gland's response. If cancer of the thyroid gland is suspected, a biopsy is performed. When medullary thyroid cancer is suspected, blood levels of calcitonin are checked, because these cancers always secrete calcitonin.

See the sidebar Euthyroid Sick Syndrome.