A T3 test helps to determine whether the thyroid is functioning properly. It is ordered primarily to help diagnose hyperthyroidism and may be ordered to help monitor the progress of a patient with a known thyroid disorder. The T3 test is usually ordered following an abnormal TSH and T4 test. Either the total T3 or the free T3 may be ordered. Since most of the T3 is bound to protein, the total T3 can be affected by protein levels and protein binding ability, but the free T3 is not. T3 testing may be ordered along with thyroid antibodies to help diagnose Graves’ disease, an autoimmune disorder that is the most common cause of hyperthyroidism.

A total or free T3 test may be ordered when a patient has an abnormal TSH and/or T4 test result. It may be ordered as part of the investigative workup when a patient has symptoms suggesting hyperthyroidism. One of the T3 tests 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 doctor specifically what is causing the excess or deficiency. The following table summarizes 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; Rare pituitary (secondary) hypothyroidism

If a patient is being treated with anti-thyroid medication for hyperthyroidism and the T3 (or more frequently, the T4 or TSH) is normal, then it is likely that the medication is controlling the condition. If the T3 (or T4) is elevated, then the medication is not sufficient to control the condition and the patient may be experiencing symptoms associated with hyperthyroidism.

Many medications—including estrogen, certain types of birth control pills, and large doses of aspirin—can affect total T3 test results, so tell your doctor about any drugs you are taking. In general, free T3 levels are not affected by these medications.

When you are sick, your 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, doctors do not usually use T3 as a routine thyroid test for patients in hospitals.