Total T4 and free T4 are two separate tests that can help the doctor evaluate thyroid function. The total T4 test has been used for many years to help diagnose hyper- and hypothyroidism. It is a useful test but can be affected by the amount of protein available in the blood to bind to the hormone. The free T4 test is a newer test that is not affected by protein levels. Since free T4 is the active form of thyroxine, it 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. Whichever thyroxine measurement is ordered, it is usually ordered along with or following a TSH test. This helps the doctor to determine whether the thyroid hormone feedback system is functioning as it should, and the results of the tests help to distinguish between different causes of hyper- and hypothyroidism. Sometimes a T3 test will also be ordered to give the doctor additional diagnostic information.

A T4 and TSH test may be ordered to help evaluate a patient with an enlarged thyroid gland (a goiter) and to aid in the diagnosis of female infertility. If the doctor suspects that the patient may have an autoimmune-related thyroid condition, thyroid antibodies may be ordered along with a T4. In patients with known thyroid dysfunction, T4 and/or TSH tests may be ordered to monitor thyroid function.

In the United States, newborns are commonly screened for T4 levels as well as TSH concentrations to check for congenital hypothyroidism, which can cause mental retardation if left untreated.

A total T4 or free T4 test is primarily ordered in response to an abnormal TSH test result. Sometimes the T4 will be ordered along with a TSH to give the doctor a more complete evaluation of the adequacy of the thyroid hormone feedback system. These tests are usually ordered when a patient has symptoms of hyper- or hypothyroidism. Sometimes they are ordered as screening tests during routine blood testing, but expert opinions vary on who can benefit from screening and at what age to begin. In patients with known thyroid disorders, the TSH and sometimes T4 are ordered at intervals to monitor the effectiveness of treatment. In pregnant women with thyroid disorders, the doctor is likely to order thyroid testing early and late in the pregnancy and for a time period following delivery to monitor the mother and baby. Thyroid hormone screening is commonly performed on newborns in the United States as part of newborn screening programs.

In general, high free or total T4 results may indicate an overactive thyroid gland (hyperthyroidism), and low free or total T4 results may indicate an underactive thyroid gland (hypothyroidism). The test results alone are not diagnostic but will prompt the doctor to perform additional testing to investigate the cause of the excess or deficiency. Both decreased and increased T4 results are associated with a variety of temporary and chronic thyroid conditions. Low T4 results in conjunction with a low TSH level or high T4 results along with a high TSH may indicate a pituitary gland condition.

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

Many medications—including estrogen, certain types of birth control pills, and large doses of aspirin—can affect total T4 test results, so tell your doctor about any drugs you are taking. In general, free T4 levels are not affected by these medications.