Thyroid Panel

How is it used?

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:

• 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?

A thyroid panel may be ordered 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
• Menstrual irregularity in women
   

Signs and symptoms of hyperthyroidism may include:

• Increased heart rate
• Anxiety
• Weight loss
• Difficulty sleeping
• Tremors in the hands
• Weakness
• Diarrhea
• 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.

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:

• Increases, decreases, and changes (inherited or acquired) in the proteins that bind T4 and T3
• Pregnancy
• Estrogen and other drugs
• Liver disease
• Systemic illness
• Resistance to thyroid hormones
• Pituitary dysfunction

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?

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.