A blood sample drawn from a vein in your arm
None needed; however, certain medications can interfere with the tests included in the panel, so tell your health practitioner about any drugs that you are taking.
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:
- TSH (thyroid-stimulating hormone) – to test for hypothyroidism, hyperthyroidism and to monitor treatment for a thyroid disorder
- Free T4 (thyroxine) – to test for hypothyroidism and hyperthyroidism; may also be used to monitor treatment
- Free T3 or total T3 (triiodothyronine) – to test for hyperthyroidism; may also be used to monitor treatment
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.
How is the sample collected for testing?
A blood sample is obtained by inserting a needle into a vein in the arm.
Is any test preparation needed to ensure the quality of the sample?
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.
How is it used?
- Thyroid-stimulating hormone (TSH)
- Free thyroxine (free T4)
- Total or free triiodothyronine (total or free T3)
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.
When is it ordered?
- Weight gain
- Dry skin
- Cold intolerance
- Puffy skin
- Hair loss
- Menstrual irregularity in women
Signs and symptoms of hyperthyroidism may include:
- Increased heart rate
- Weight loss
- Difficulty sleeping
- Tremors in the hands
- Puffiness around the eyes, dryness, irritation, or bulging of the eyes
What does the test result mean?
If the feedback system involving the thyroid gland is not functioning properly due to one of a variety of disorders, then increased or decreased amounts of thyroid hormones may result. When TSH concentrations are increased, the thyroid will make and release inappropriate amounts of T4 and T3 and the person may experience symptoms associated with hyperthyroidism. If there is decreased production of thyroid hormones, the person may experience symptoms of hypothyroidism.
The following table summarizes some examples of typical test results and their potential meaning.
TSH T4 T3 Interpretation High Normal Normal Mild (subclinical) hypothyroidism High Low Low or normal Hypothyroidism Low Normal Normal Mild (subclinical) hyperthyroidism Low High or normal High or normal Hyperthyroidism Low Low or normal Low or normal Nonthyroidal illness; pituitary (secondary) hypothyroidism Normal High High Thyroid hormone resistance syndrome (a mutation in the thyroid hormone receptor decreases thyroid hormone function)
The above test results alone are not diagnostic but will prompt a health practitioner to perform additional testing to investigate the cause of the excess or deficiency and thyroid disorder. As examples, the most common cause of hyperthyroidism is Graves disease and the most common cause of hypothyroidism is Hashimoto thyroiditis. (See the condition article on Thyroid Diseases for more on these and other related diseases.)
Is there anything else I should know?
In the past, panels of tests were more common. More recently, however, the practice has been to order, where possible, one initial or screening test and then follow up with additional testing, if needed, to reduce the number of unnecessary tests. With thyroid testing, one strategy is to screen with a TSH test and then order additional tests if the results are abnormal or if clinical suspicions warrant.
It is important to note that thyroid tests are a "snapshot" of what is occurring within a dynamic system. An individual person's total T4, free T4, total T3, free T3, and/or TSH results may vary and may be affected by:
What conditions are associated with hypo- and hyperthyroidism?
What other tests may be ordered in addition to a thyroid panel?
Blood tests that may be performed in addition to a thyroid panel may include:
- Thyroid antibodies - to help differentiate different types of thyroiditis and identify autoimmune thyroid conditions
- Calcitonin - to help detect the presence of excessive calcitonin production as can occur with C-cell hyperplasia and medullary thyroid cancer
- Thyroglobulin - to monitor treatment of thyroid cancer
- Thyroxine-binding globulin (TBG) - to evaluate patients with abnormal T4 and T3 levels
What is reverse T3?
Reverse T3 (RT3 or REVT3) is a biologically inactive form of T3. Normally, when T4 is converted to T3 in the body, a certain percentage of the T3 is in the form of RT3. When the body is under stress, such as during a serious illness, thyroid hormone levels may be outside of normal ranges even though there is no thyroid disease present. RT3 may be elevated in non-thyroidal conditions, particularly the stress of illness. It is generally recommended that thyroid testing be avoided in hospitalized patients or deferred until after a person has recovered from an acute illness. Use of the RT3 test remains controversial, and it is not widely requested.
On This Site
Feature Article: Biotin Affects Some Blood Test Results
Elsewhere On The Web
Hormone Health Network: Thyroid Disorders Resources
EndocrineWeb: How Your Thyroid Works
KidsHealth: Thyroid Disease
National Endocrine and Metabolic Diseases Information Service: Pregnancy and Thyroid Disease
American Thyroid Association: Patients & The Public - Thyroid Information
American Association of Clinical Endocrinologists: About Your Thyroid
MedlinePlus Medical Encyclodpedia: Thyroid Diseases