Overview of the Thyroid Gland

ByLaura Boucai, MD, Weill Cornell Medical College
Reviewed/Revised Feb 2024

The thyroid is a small gland, measuring about 2 inches (5 centimeters) across, that is located just under the skin 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 in the neck (sometimes 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 2 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

Spotlight on Aging: Thyroid Gland Changes in Older Adults

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 the thyroid hormone triiodothyronine (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 adults. 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 adult 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.

Some experts recommend measuring the level of thyroid-stimulating hormone in the blood in people over 70 years old every year or every few years, although a number of medical bodies that have examined this question recommend against screening adults who have no symptoms in order to avoid over-treating people with minor laboratory abnormalities.

Thyroid hormones

There are 2 thyroid hormones

  • T4: Thyroxine (also called tetraiodothyronine)

  • T3: Triiodothyronine

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, 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 or has bumps (nodules).

Depending on the results of the examination, other tests may also be needed.

Thyroid function blood tests

To determine how well the thyroid gland is functioning, doctors usually measure the levels of hormones in the blood. They measure levels of

  • TSH

  • T4

  • T3

Usually the level of TSH 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 usually high when the thyroid gland is underactive (hypothyroidism), and thus needs more stimulation, and low when the thyroid gland is overactive (hyperthyroidism), 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 a person is tested for a thyroid problem, TSH is always measured, and sometimes other tests are also done. Also, for people with low thyroid function (hypothyroidism) who are treated with thyroid hormone as a medication, TSH is usually measured every few months or every year to check if the dose of the medication needs to be adjusted.

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). The majority of the circulating levels of T4 and T3 are bound to a protein called thyroxine-binding globulin. 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 thyroid hormones in the blood (free T4 and free T3). 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.

Thyroid ultrasound

If a doctor feels one or more lumps (nodules) in the thyroid gland, an ultrasound 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), what the characteristics of the nodule are such as the presence or absence of calcium, as well as the number and prominence of blood vessels in the thyroid gland.

In another test (called a radioactive iodine uptake test—a type of radionuclide scan), 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, a thyroid scan 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, a blood test is done to look for antibodies that are attacking the thyroid 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). 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 recommend screening for thyroid disease by measuring the level of TSH in the blood every year or every few years in people older than 70 years old. However, a number of professional medical societies that have examined this question recommend against screening adults who have no symptoms in order to avoid over-treating people with minor laboratory abnormalities. Screening is recommended for all newborns to detect hypothyroidism (congenital hypothyroidism), which can cause major defects in development of the brain and other organs if left untreated.

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