The thyroid is a small gland, measuring about 2 inches (5 centimeters) 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. If it becomes enlarged, doctors can feel it easily, and a prominent bulge (goiter) may appear below or to the sides of the Adam's apple.

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.

Locating the Thyroid Gland

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 illnesses. Most of the T₄ and T₃ in the bloodstream is carried bound to a protein called thyroxine-binding globulin. Only a little of the T₄ and T₃ are circulating free in the blood. However, it is this free hormone that is active. When the free hormone is used by the body, some of the bound hormone is released from the binding protein.

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, some 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.

Spotlight on Aging
Aging itself has only minor effects on the thyroid gland and thyroid hormones. As people get older, the thyroid gland shrinks and shifts lower in the neck. The level of T3 may fall slightly, but the speed of vital functions changes very little. However, thyroid disorders become more common with aging. Disorders that affect thyroid function, particularly hyperthyroidism and hypothyroidism, can be thought of as great masqueraders in older people. These disorders often cause symptoms that are easily mistaken for symptoms of other conditions or even as signs of getting old. Increased or decreased thyroid function can dramatically worsen the way an older person feels and can greatly diminish the ability to carry out daily activities. For these reasons, the great masqueraders must be unmasked and recognized for what they are so that they can be effectively treated. Screening older people for hyperthyroidism and hypothyroidism is helpful. Many experts recommend measuring the level of thyroid-stimulating hormone in the blood every year in people over 65.

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 usually measure the levels of TSH, T₄, and T₃ in the blood.

Usually the TSH level is the best indicator of thyroid function. 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.

When doctors measure the levels of thyroid hormones T₄ and T₃ in the blood, they usually measure both the bound and free forms of each hormone (total T₄ and total T₃). However, if the level of thyroxine-binding globulin is abnormal, the total thyroid hormone levels can be misinterpreted, so doctors sometimes measure the level of free hormones in the blood. The level of thyroxine-binding globulin is lower in people who have kidney disease or diseases that reduce protein synthesis by the liver 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, a scanning procedure may be done. Ultrasonography uses sound waves to measure the size of the gland and to determine whether the growth is solid or filled with fluid (cystic). In another type of thyroid scan, a small amount of radioactive iodine or technetium is injected into the bloodstream. The radioactive material concentrates in the thyroid gland and a device (gamma camera) that detects the radiation produces 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.

Euthyroid Sick Syndrome
In euthyroid sick syndrome, thyroid test results are abnormal even though the thyroid gland is functioning normally. Euthyroid sick syndrome commonly occurs in people who have a severe illness other than thyroid disease. When people are sick or undernourished or have had surgery, the thyroid hormone T4 is not converted normally to the active T3 hormone. Large amounts of reverse T3, an inactive thyroid hormone, accumulate. Despite this abnormal conversion, the thyroid gland continues to function and to control the body's metabolic rate normally. Because no problem exists with the thyroid gland, no treatment is needed. Laboratory tests show normal results once the underlying illness resolves.

Additional testing may be necessary in rare cases in which doctors cannot determine whether the problem lies in the thyroid or in the pituitary gland.

If cancer of the thyroid gland is suspected, doctors use a small needle to obtain a sample of thyroid tissue for study (a biopsy). When medullary thyroid cancer is suspected, blood levels of calcitonin are measured, because these cancers always secrete calcitonin.

Last full review/revision May 2012 by Jerome M. Hershman, MD