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Thyroid Panel

What is a thyroid panel?

A thyroid panel is a group of tests that are often 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 thyroidhormones in your blood. These hormones are chemical substances that travel through the bloodstream and control or regulate your body's metabolism—how it functions and uses energy.

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 concentrations decrease in the blood, the pituitary is stimulated to release TSH. The 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.

The thyroid panel usually includes:

  • TSH (thyroid stimulating hormone or thyrotropin)– to test for hypothyroidism, hyperthyroidism and to monitor thyroid replacement therapy

  • T4 or free T4 (thyroxine)– to test for hypothyroidism and hyperthyroidism

  • T3 or free T3 (triiodothyronine)– to test for hyperthyroidism

Sometimes a T3 uptake test is included to calculate free thyroxine index (FTI).

How is a thyroid panel used?

A thyroid panel is used to screen for or help diagnose hypo- and hyperthyroidism due to various thyroid disorders.

The preferred test to screen for thyroid disorders is a TSH test. If your TSH level is abnormal, it will usually be followed up with a test for total T4 or free T4. Sometimes a total T3 or free T3 will also be performed. Often, the laboratory will do this follow-up testing automatically, and this is known as reflex testing.  This saves your doctor time from having to wait for the results of the initial test and then requesting the additional testing to confirm or clarify a diagnosis.  Follow-up tests are often performed on the original sample that was submitted when the initial test was requested. A thyroid panel may be requested by your doctor to have all three tests performed at the same time to get a more complete picture.

When is it ordered?

A thyroid panel may be ordered as part of a health checkup or when symptoms suggest hypo- or hyperthyroidism due to a condition affecting the thyroid.

Signs and symptoms of hypothyroidism may include weight gain, dry skin, constipation, cold intolerance, puffy skin, hair loss, fatigue, and menstrual irregularity in women.

Signs and symptoms of hyperthyroidism may include increased heart rate, anxiety, weight loss, difficulty sleeping, tremors in the hands, weakness, and sometimes diarrhea. There may be puffiness around the eyes, dryness, irritation, and, in some cases, bulging of the eyes.

What do the test results 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 patient may experience symptoms associated with hyperthyroidism. If there is decreased production of thyroid hormones, the patient may experience symptoms of hypothyroidism.

The following table summarizes test results and their potential meaning.

TSH

T4

T3

INTERPETATION

High

Normal

Normal

Mild (subclinical) hypothyroidism

High

Low

Low or normal

Hypthyroidism

Low

Normal

Normal

Mild (subclinical) hyperthyroidism

Low

High or normal

High or normal

Hyperthyroidism

Low

Low or normal

Low or normal

Nonthyroidal illness; rare pituitary (secondary) hypothyroidism

Is there anything else I should know?

In the past, panels of tests were more common.  More recently, however, the practice is 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, if the results are abnormal, then order additional tests.  Many laboratories still perform thyroid panels, but this will become less common in the future as better screening strategies are developed.

Common Questions

What conditions are associated with hypo- and hyperthyroidism?

The most common causes of thyroid dysfunction are autoimmune-related. Graves disease causes hyperthyroidism, and Hashimoto thyroiditis causes hypothyroidism. Both hyper- and hypothyroidism can also be caused by thyroiditis, thyroid cancer, and excessive or deficient production of TSH.

What other tests may be ordered in addition to a thyroid panel?

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

  • 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 illnesses; thus, the RT3 test may sometimes be used to help determine if stress of illness is a cause of thyroid hormone changes. However, it is generally recommended that thyroid testing be avoided or deferred until after a patient has recovered from an acute illness. Use of the RT3 test remains controversial and it is not widely requested.

Content last reviewed in July 2010. | This page was last modified in December 2010.

Content by Lab Tests Online

 

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