Free T3

A free or total triiodothyronine (free T3 or total T3) test is used to assess thyroid function. It is ordered primarily to help diagnose hyperthyroidism and may be ordered to help monitor treatment of a person with a known thyroid disorder.

T3 and T4 (thyroxine) 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. Thyroid-stimulating hormone (TSH) stimulates the production and release of T4 (primarily) and T3. As needed, T4 is converted into T3 by the liver and other tissues.

Most of the T4 and T3 circulates in the blood bound to protein, while a small percentage is free (not bound). Blood tests can measure total T4 (unbound plus bound), free T4, total T3 (bound plus unbound), or free T3.

Since most T3 is bound to protein, the total T3 can be affected by protein levels and protein binding ability, but the free T3 is not. However, some professional guidelines recommend the total T3, so either test may be used to assess thyroid function. For example, free T3 or sometimes total T3 may be ordered along with thyroid antibodies to help diagnose Graves disease, an autoimmune disorder that is the most common cause of hyperthyroidism.

The free or total T3 test is usually ordered following an abnormal TSH, particularly if the free T4 test is not elevated.

A free T3 or total T3 test may be ordered when someone has an abnormal TSH test result. It may be ordered as part of the investigative workup when a person has symptoms suggesting hyperthyroidism, especially if the free T4 level is not elevated.

Signs and symptoms may include:

• Increased heart rate
• Anxiety
• Weight loss
• Difficulty sleeping
• Tremors in the hands
• Weakness
• Diarrhea (sometimes)
• Light sensitivity, visual disturbances
• The eyes may be affected: puffiness around the eyes, dryness, irritation, and, in some cases, bulging of the eyes.

Free or total T3 may sometimes be ordered at intervals to monitor a known thyroid condition and to help monitor the effectiveness of treatment for hyperthyroidism.

Increased or decreased thyroid hormone results indicate that there is an imbalance between the body's requirements and supply, but they do not tell the health practitioner specifically what is causing the excess or deficiency.

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

If someone is being treated with anti-thyroid medication for hyperthyroidism and the free or total T3 (or more frequently, the free T4 or TSH) is normal, then it is likely that the medication is effective in treating the condition. If the free or total T3 or free T4 is elevated, then the medication is not effective in treating the condition and the person may be experiencing symptoms associated with hyperthyroidism.

It is generally recommended that thyroid testing be avoided in hospitalized patients or deferred until after a person has recovered from an acute illness since thyroid hormone levels may be affected the stress of an illness. When someone is sick, the body decreases production of T3 from T4. Most people who are sick enough to be in the hospital will have a low T3 or free T3 level. For this reason, health practitioners usually only order T3 tests in outpatient settings.

It is important to note that thyroid tests are a "snapshot" of what is occurring within a dynamic system. An individual person's total T3, free T3, total T4, free T4, 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

Many medications—including estrogen, certain types of birth control pills, and large doses of aspirin—can affect total T3 test results and their use should be discussed with a health practitioner prior to testing. In general, free T3 levels are not affected by these medications.