January 1, 2015
What is the thyroid?
The thyroid is a 2-inch-long, butterfly-shaped gland weighing less than 1 ounce. Located in the front of the neck below the larynx, or voice box, it has two lobes, one on either side of the windpipe.
The thyroid is one of the glands that make up the endocrine system. The glands of the endocrine system produce and store hormones and release them into the bloodstream. The hormones then travel through the body and direct the activity of the body’s cells.
What is the role of thyroid hormones?
Thyroid hormones regulate metabolism—the way the body uses energy—and affect nearly every organ in the body. Thyroid hormones also affect brain development, breathing, heart and nervous system functions, body temperature, muscle strength, skin dryness, menstrual cycles, weight, and cholesterol levels.
The thyroid makes two thyroid hormones:
Only a small amount of T3 in the blood comes from the thyroid. Most T3 comes from cells all over the body, where it is made from T4. Thyroid-stimulating hormone (TSH), which is made by the pituitary gland in the brain, regulates thyroid hormone production. When thyroid hormone levels in the blood are low, the pituitary releases more TSH. When thyroid hormone levels are high, the pituitary decreases TSH production. When thyroid hormone levels in the blood are low, the pituitary releases more TSH. When thyroid hormone levels are high, the pituitary decreases TSH production.
Why do health care providers perform thyroid tests?
Health care providers perform thyroid tests to assess how well the thyroid is working. The tests are also used to diagnose and help find the cause of thyroid disorders such as hyperthyroidism and hypothyroidism:
- Hyperthyroidism is a disorder caused by too much thyroid hormone in the bloodstream, which increases the speed of bodily functions and leads to weight loss, sweating, rapid heart rate, and high blood pressure, among other symptoms.
- Hypothyroidism is a disorder that occurs when the thyroid doesn’t make enough thyroid hormone for the body’s needs. Without enough thyroid hormone, many of the body’s functions slow down. People may have symptoms such as fatigue, weight gain, and cold intolerance.
What blood tests do health care providers use to check a person’s thyroid function?
A health care provider may order several blood tests to check thyroid function, including the following:
- TSH test
- T4 tests
- T3 test
- thyroid-stimulating immunoglobulin (TSI) test
- antithyroid antibody test, also called the thyroid peroxidase antibody test (TPOab)
A blood test involves drawing blood at a health care provider’s office or a commercial facility and sending the sample to a lab for analysis. Blood tests assess thyroid function by measuring TSH and thyroid hormone levels, and by detecting certain autoantibodies present in autoimmune thyroid disease. Autoantibodies are molecules produced by a person’s body that mistakenly attack the body’s own tissues.
Many complex factors affect thyroid function and hormone levels. Health care providers take a patient’s full medical history into account when interpreting thyroid function tests.
A health care provider usually performs the TSH blood test first to check how well the thyroid is working. The TSH test measures the amount of TSH a person’s pituitary is secreting. The TSH test is the most accurate test for diagnosing both hyperthyroidism and hypothyroidism. Generally, a below-normal level of TSH suggests hyperthyroidism. An abnormally high TSH level suggests hypothyroidism.
The TSH test detects even tiny amounts of TSH in the blood. Normally, the pituitary boosts TSH production when thyroid hormone levels in the blood are low. The thyroid responds by making more hormone. Then, when the body has enough thyroid hormone circulating in the blood, TSH output drops. The cycle repeats continuously to maintain a healthy level of thyroid hormone in the body. In people whose thyroid produces too much thyroid hormone, the pituitary shuts down TSH production, leading to low or even undetectable TSH levels in the blood.
In people whose thyroid is not functioning normally and produces too little thyroid hormone, the thyroid cannot respond normally to TSH by producing thyroid hormone. As a result, the pituitary keeps making TSH, trying to get the thyroid to respond.
If results of the TSH test are abnormal, a person will need one or more additional tests to help find the cause of the problem.
The thyroid primarily secretes T4 and only a small amount of T3. T4 exists in two forms:
- T4 that is bound to proteins in the blood and is kept in reserve until needed
- a small amount of unbound or “free” T4 (FT4), which is the active form of the hormone and is available to enter body tissues when needed
A high level of total T4—bound and FT4 together—or FT4 suggests hyperthyroidism, and a low level of total T4 or FT4 suggests hypothyroidism.
Both pregnancy and taking oral contraceptives increase levels of binding protein in the blood. In either of these cases, although a woman may have a high total T4 level, she may not have hyperthyroidism. Severe illness or the use of corticosteroids—a class of medications that treat asthma, arthritis, and skin conditions, among other health problems—can decrease binding protein levels. Therefore, in these cases, the total T4 level may be low, yet the person does not have hypothyroidism.
If a health care provider suspects hyperthyroidism in a person who has a normal FT4 level, a T3 test can be useful to confirm the condition. In some cases of hyperthyroidism, FT4 is normal yet free T3 (FT3) is elevated, so measuring both T4 and T3 can be useful if a health care provider suspects hyperthyroidism. The T3 test is not useful in diagnosing hypothyroidism because levels are not reduced until the hypothyroidism is severe.
Thyroid-stimulating immunoglobulin is an autoantibody present in Graves’ disease. TSI mimics TSH by stimulating the thyroid cells, causing the thyroid to secrete extra hormone. The TSI test detects TSI circulating in the blood and is usually measured
- in people with Graves’ disease when the diagnosis is obscure
- during pregnancy
- to find out if a person is in remission, or no longer has hyperthyroidism and its symptoms
Antithyroid antibodies are markers in the blood that are extremely helpful in diagnosing Hashimoto’s disease. Two principal types of antithyroid antibodies are
- anti-TG antibodies, which attack a protein in the thyroid called thyroglobulin
- anti-thyroperoxidase, or anti-TPO, antibodies, which attack an enzyme in thyroid cells called thyroperoxidase
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