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
By increasing the amount of oxygen that cells use
Thyroid hormones affect many vital body functions, such as the heart 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 (also called tetraiodothyronine)
T4, the major hormone produced by the thyroid gland, has only a slight effect, if any, 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 Iodine Deficiency Iodine deficiency, which is common worldwide, can lead to enlargement of the thyroid gland. The thyroid gland contains most of the iodine in the body. Iodine in the thyroid gland is necessary... read more , 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.
The thyroid gland also produces the hormone calcitonin, which may contribute to bone strength by helping calcium to be incorporated into bone.
How the body adjusts thyroid hormones
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.
Diagnostic Tests of the Thyroid Gland
Doctors first examine the person and feel the person's neck to see whether the thyroid gland is enlarged.
Depending on the results of the examination, other tests may also be needed. Additional testing may also be necessary in rare cases in which doctors cannot determine whether the problem lies in the thyroid or in the pituitary gland Overview of the Pituitary Gland The pituitary is a pea-sized gland that is housed within a bony structure (sella turcica) at the base of the brain. The sella turcica protects the pituitary but allows very little room for expansion... read more .
Thyroid function tests
To determine how well the thyroid gland is functioning, doctors usually measure the levels of hormones in the blood. They measure levels of
Usually the level of thyroid-stimulating hormone in the blood is the best indicator of thyroid function. Because the role of this hormone is to stimulate 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 only 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 the amount of protein made 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 Overview of Acute Viral Hepatitis Acute viral hepatitis is inflammation of the liver caused by infection with one of the five hepatitis viruses. In most people, the inflammation begins suddenly and lasts only a few weeks. Symptoms... read more .
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 test (called a radioactive iodine uptake test—a type of radionuclide scan Radionuclide Scanning In radionuclide scanning, radionuclides are used to produce images. A radionuclide is a radioactive form of an element, which means it is an unstable atom that becomes more stable by releasing... read more ), a small amount of a radioactive substance (such as iodine or technetium) is injected into the bloodstream. The radioactive material concentrates in the thyroid gland, and a device (gamma camera) does another type of scan that detects the radiation and produces a picture of the thyroid gland that shows any physical abnormalities.
Because the thyroid gland takes up radioactive iodine depending on how well it is working, 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.
Other thyroid tests
If doctors suspect an autoimmune disorder Autoimmune Disorders An autoimmune disorder is a malfunction of the body's immune system that causes the body to attack its own tissues. What triggers autoimmune disorders is not known. Symptoms vary depending on... read more , a blood test is done to look for antibodies that are attacking the thyroid gland.
If cancer of the thyroid gland Thyroid Cancer The cause of thyroid cancer is not known, but the thyroid gland is very sensitive to radiation, which may cause cancerous changes. Thyroid cancer is more common among people who were treated... read more is suspected, doctors use a small needle to obtain a sample of thyroid tissue for study (a biopsy). Doctors usually use ultrasonography to identify the site to biopsy.
When medullary thyroid cancer is suspected, blood levels of calcitonin are measured, because these cancers always secrete calcitonin.
Screening for thyroid disorders
Some experts disagree about whether people should be screened for thyroid disorders (screening Screening tests Because many different diseases can cause the same symptoms, it can be challenging for doctors and other primary care practitioners to identify the cause. Doctors first gather basic information... read more is doing tests to look for a disorder in people who have no symptoms). Some experts do not recommend screening, saying that if thyroid disease does not cause symptoms, it is not clear that treatment is beneficial. However, other experts disagree. They recommend screening because thyroid disease is common, especially among older people, and can cause serious problems. Screening every 5 years by measuring the level of thyroid stimulating hormone in the blood is recommended for women 50 and older and men 65 and older. Screening is also recommended for all newborns and for pregnant women.