The thyroid—a butterfly-shaped endocrine gland located at the front of the neck—is essential for growth and development in children and is the primary regulator of body metabolism. The functions of the thyroid—which include maintaining the correct metabolic rate for cells to function and helping to control the body’s use of food for energy—are primarily carried out by two major hormones, T3 and T4. Blood tests to measure thyroid hormones are most useful for evaluating disorders of thyroid function, including overactivity (hyperthyroidism) and underactivity (hypothyroidism).
Thyroxine (T4) controls body metabolism, helps to oversee physical and mental development, and plays a part in resistance to infection and vitamin requirements. It is secreted by the thyroid in response to the release of thyroid-stimulating hormone by the pituitary gland.
Triiodothyronine (T3)—a more potent molecule than T4—is essential for maintaining the rate of chemical reactions, or metabolic rate, in all cells in the body. Most T3 is produced by tissues throughout the body (primarily the liver) through the removal of an iodine atom from T4; the rest is secreted directly by the thyroid. Although T3 is present in smaller quantities and is metabolically active for a shorter time than T4, its impact on body metabolism is of greater magnitude.
Thyroxine-binding globulin (TBG) is the major thyroid hormone protein carrier. Most of the T4 and T3 in the blood is bound to proteins, primarily TBG; the remaining fraction, which circulates freely and thus is known as free T4 and T3, makes up the active portion of these hormones. When TBG levels are elevated—as in pregnancy or in those taking estrogen replacement therapy or oral contraceptives—less free (and active) T3 and T4 is available. To compensate, the thyroid responds by releasing more T3 and T4. By measuring TBG, it is possible to determine whether abnormal T4 and T3 levels are the result of this normal response or whether there is true thyroid dysfunction.
The T3 uptake test provides an indirect measurement of T4 levels, as well as the amount of available protein (primarily TBG) that can bind to T3 and T4. In T3 uptake, radioactive T3 (RT3) is added your blood sample in the laboratory; by measuring how much RT3 can attach to thyroid hormone-binding proteins, the amount of T4 that is already bound to the proteins can be indirectly quantified. The results of T3 uptake are only useful when considered together with other thyroid hormone tests.
Thyroid-stimulating hormone (TSH), also known as thyrotropin, is a hormone secreted by the pituitary gland in the brain in response to hormonal signals from the hypothalamus (another part of the brain). The function of TSH is to stimulate the release of both T4 and T3 by the thyroid gland. Problems with TSH secretion lead to abnormal T4 and T3 levels, and thus result in thyroid dysfunction.
Purpose of the Thyroid Panel
T4, T3, and TSH:
- To evaluate thyroid function
- To help diagnose hyperthyroidism (particularly in people with typical symptoms such as rapid heartbeat, weight loss, and dizziness) and hypothyroidism (which causes symptoms such as fatigue, sensitivity to cold, and weight gain)
- To monitor the effectiveness of drug therapy for thyroid dysfunction
- T4 screens newborn babies for hypothyroidism.
- TSH helps to distinguish between primary (originating in the thyroid) and secondary (originating in the pituitary or hypothalamus) hypothyroidism.
- TSH screens for and helps diagnose thyroid disorders and monitors treatment for hypothyroidism and hyperthyroidisim.
TBG and T3 uptake:
- To evaluate thyroid function and aid in the assessment of patients with abnormal T3 and T4 levels
Who Performs Thyroid Biopsy
- A nurse or a technician will draw the blood sample
Special Concerns about Thyroid Panel
- A variety of medications and hormone supplements may interfere with the results of these tests. In addition, pregnancy causes increased levels of TBG, T4, and total (but not free) T3.
- Because TSH levels vary throughout the day—generally peaking at around midnight—this test is scheduled for a particular time of day, usually in the morning.
- A recent nuclear scanning test may affect the results of certain thyroid hormone tests, since these chemicals may be measured with a laboratory technique that utilizes a radioactive isotope (radioimmunoassay).
Before the Thyroid Panel
- Report to your doctor any medications, herbs, or supplements you are taking. You may be advised to discontinue certain of these agents before the test.
- Tell your doctor if you’ve had any recent procedures, such as a nuclear scan, that introduced radioactive material into your bloodstream.
What You Experience
- A sample of your blood is drawn from a vein, usually in your arm, and sent to a laboratory for analysis.
- For a TSH test, the blood sample is typically drawn in the morning.
Risks and Complications
After the Thyroid Panel
- Immediately after blood is drawn, pressure is applied (with cotton or gauze) to the puncture site.
- Resume your normal activities and any medications withheld before the test.
- Blood may collect and clot under the skin (hematoma) at the puncture site; this is harmless and will resolve on its own. For a large hematoma that causes swelling and discomfort, apply ice initially; after 24 hours, use warm, moist compresses to help dissolve the clotted blood.
Results of Thyroid Panel
- Your blood sample is sent to a laboratory for analysis. The doctor will review the results for evidence of a thyroid disorder.
- Elevated T3 and T4 levels indicate hyperthyroidism, while low levels of these hormones suggest hypothyroidism. Abnormal TSH levels suggest that thyroid dysfunction results from problems originating in the pituitary gland or hypothalamus.
- If an abnormality is found and the doctor can make a definitive diagnosis, appropriate treatment will begin.
- In some cases, abnormal results on one or more of the thyroid hormone tests will necessitate additional tests—such as a thyroid nuclear scan—to establish a diagnosis.
The Johns Hopkins Consumer Guide to Medical Tests
Simeon Margolis, M.D., Ph.D., Medical Editor
Updated by Remedy Health Media