Thyroid Panel

How is the test used?

A thyroid panel may be used to evaluate thyroid function and/or help diagnose thyroid disorders.

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 healthcare 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 your healthcare 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?

A thyroid panel may be ordered when you have signs and symptoms that suggest underactive thyroid (hypothyroidism) or overactive thyroid (hyperthyroidism) due to a thyroid disorder.

Signs and symptoms of an underactive thyroid may include:

• Slowed heart rate
• Weight gain
• Enlarged thyroid (goiter)
• Dry skin
• Constipation
• Trouble tolerating cold
• Muscle and joint pain
• Puffy skin
• Thinning hair, hair loss
• Fatigue
• Depression
• Forgetfulness
• Heavy or irregular menstrual periods or infertility in women

Signs and symptoms of an overactive thyroid may include:

• Rapid heart rate
• Anxiety
• Weight loss
• Difficulty sleeping
• Tremors in the hands
• Muscle weakness
• Increased sweating
• Trouble tolerating heat
• Sometimes more frequent bowel movements
• Some uncommon problems that can affect the eyes: puffiness around the eyes, dryness, irritation, excessive tearing, light sensitivity, blurry double vision
• In some cases, bulging of the eyes
• Less frequent or lighter menstrual periods in women

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 levels are increased, the thyroid will make and release inappropriate amounts of T4 and T3 and you may experience symptoms associated with hyperthyroidism.
• If there is decreased production of thyroid hormones, you may have symptoms of hypothyroidism.

The test results alone are not diagnostic but will prompt your healthcare 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.)

The following table summarizes some examples of typical test results and their potential meaning.

¹In affected adults, the diagnosis of subclinical hypothyroidism is applied when the TSH level is elevated and the free T4 level is normal on repeat testing over a number of weeks or months. Adults with subclinical hypothyroidism may have few or no overt symptoms of hypothyroidism. However, subclinical hypothyroidism places affected adults at somewhat increased risk for an elevated LDL cholesterol level, increased risk for cardiovascular disease, and reduced mental acuity.

²In affected adults, the diagnosis of subclinical hyperthyroidism is applied when the TSH level is decreased and the free T4 level and T3 levels are normal on repeat testing over a number of weeks or months. Adults with subclinical hyperthyroidism may have few or no overt symptoms of hyperthyroidism. However, subclinical hyperthyroidism places affected persons at somewhat increased risk for atrial fibrillation and osteoporosis.

What conditions are associated with hypothyroidism and hyperthyroidism?

The most common causes of thyroid dysfunction are autoimmune diseases. Graves disease causes hyperthyroidism and Hashimoto thyroiditis causes hypothyroidism. Both hyperthyroidism and hypothyroidism can also be caused by thyroiditis, thyroid cancer, and too much or too little TSH.

What medications can affect the thyroid panel tests?

Many multivitamins, supplements (especially hair and nail), and over-the-counter and prescription medications may affect thyroid test results and their use should be discussed with your healthcare practitioner prior to testing. For example, biotin (vitamin B7) can interfere with some lab tests, so your healthcare practitioner may advise you to refrain from taking biotin or supplements that contain biotin for a few days before having blood drawn for a thyroid panel.

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 diagnose autoimmune thyroid disease and distinguish it from other thyroid conditions (e.g., Graves disease, Hashimoto thyroiditis)
• Calcitonin - to help diagnose C-cell hyperplasia and medullary thyroid cancer
• Thyroglobulin - primarily to monitor treatment and recurrence 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.

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.

Your thyroid hormone test results can be affected by:

• Increases, decreases, and changes (inherited or acquired) in the proteins that bind T4 and T3 (This is important for tests that measure total T3, but it is not likely important for free T4 and free T3 tests, which measure the thyroid hormones that are not bound to protein.)
• Pregnancy
• Estrogen and other drugs
• Liver disease
• Systemic illness
• Rarely, resistance to thyroid hormones