A thyroid panel is a group of tests that may be ordered together to help evaluate thyroid gland function and to help diagnose thyroid disorders. The tests included in a thyroid panel measure the amount of thyroid hormones in the blood. These hormones are chemical substances that travel through the blood and control or regulate the body's metabolism—how it functions and uses energy.
The thyroid panel usually includes:
Sometimes a T3 resin uptake (T3RU) test is included to calculate, along with the T4 value, the free thyroxine index (FTI), another method for evaluating thyroid function that corrects for changes in certain proteins that can affect total T4 levels.
TSH is produced by the pituitary gland and is part of the body's feedback system to maintain stable amounts of the thyroid hormones T4 and T3 in the blood. When thyroid hormone levels decrease, the pituitary is stimulated to release TSH. TSH in turn stimulates the production and release of T4 and T3 by the thyroid gland. When the system is functioning normally, thyroid production turns on and off to maintain constant blood thyroid hormone levels.
T3 and T4 are the two major hormones produced by the thyroid gland, a small butterfly-shaped organ that lies flat across the windpipe at the base of the throat. Together they help control the rate at which the body uses energy. Almost all of the T3 and T4 circulating in the blood is bound to protein. The small portions that are not bound or "free" are the biologically active forms of the hormones. Tests can measure the amount of free T3 or free T4 or the total T3 or total T4 (bound plus unbound) in the blood.
The total T4 and total T3 tests have been used for many years, but they can be affected by the amount of protein available in the blood to bind to the hormone. The free T4 and free T3 tests are not affected by protein levels and are thought by many to be more accurate reflections of thyroid hormone function. In most cases, the free T4 test has replaced that of the total T4 test. However, some professional guidelines recommend the total T3 test, so either total T3 or free T3 test may be used to assess thyroid function.
A blood sample is obtained by inserting a needle into a vein in the arm.
No test preparation is needed. Certain medications can interfere with the tests included in the panel, however, so tell the health practitioner about any drugs being taken.
A thyroid panel is used to evaluate thyroid function and/or help diagnose hypothyroidism and hyperthyroidism due to various thyroid disorders. The panel typically includes tests for:
T4 and T3 are hormones produced by the thyroid gland. They help control the rate at which the body uses energy, and are regulated by a feedback system. TSH from the pituitary gland stimulates the production and release of T4 (primarily) and T3 by the thyroid. Most of the T4 and T3 circulate in the blood bound to protein. A small percentage is free (not bound) and is the biologically active form of the hormones.
Laboratory tests can measure either total amount of hormone (bound plus unbound) or just the free portion. The free T4 test is thought by many to be a more accurate reflection of thyroid hormone function and, in most cases, its use has replaced that of the total T4 test. However, some professional guidelines still recommend the total T3 test so either free T3 or total T3 may be ordered by a health practitioner.
Typically, the preferred initial test for thyroid disorders is a TSH test. If the TSH level is abnormal, it will usually be followed up with a test for free T4. Sometimes a total T3 or free T3 will also be performed. Often, the laboratory will do this follow-up testing automatically. This is known as reflex testing and it saves the health practitioner time from having to wait for the results of the initial test and then requesting additional testing to confirm or clarify a diagnosis. Reflex tests are typically performed on the original sample that was submitted when the initial test was requested.
As an alternative, a thyroid panel may be requested by the health practitioner. This means that all three tests will be performed at the same time to get a more complete initial picture of thyroid function.