Total T4 (Thyroxine)

What is Total T4?

T4 (thyroxine) is the hormone the thyroid gland produces in the greatest amount, and in the tissues it is converted, as needed, into T3, which is more active. It is the main source of the thyroid hormones that regulate the body's energy use, growth and development. Most of the thyroid hormone circulating in the blood is in the form of T4. Almost all of this hormone circulates bound to carrier proteins, mainly TBG (thyroxine-binding globulin).

The Total T4 test measures the protein-bound fraction and the free fraction together. For this reason, the result is affected not only by how the thyroid gland is working but also by the amount of carrier proteins. Although free T4 is mostly preferred for routine assessment today, total T4 gives the doctor additional information in certain situations where free T4 measurements become less reliable.

When is the Total T4 test ordered?

Total T4 is usually ordered together with TSH and free T4 to complete the picture. Because free T4 measurements can be less reliable in the later stages of pregnancy, some doctors prefer to measure total T4, using an assessment adapted to pregnancy. It can also be used for confirmation when the free T4 result does not match the symptoms. In people with a congenital excess or deficiency of carrier proteins, evaluating the free and total values together clarifies the picture.

• Symptoms of an underactive or overactive thyroid gland
• Assessment of thyroid function during pregnancy
• A mismatch between free T4 and symptoms
• Suspected carrier protein disorder
• Monitoring treatment of thyroid disease

How do you prepare for the Total T4 test?

You do not need to fast for Total T4. If you take thyroid hormone medication, taking it after your blood draw, unless your doctor advises otherwise, prevents the medication from making the result appear higher than it really is. Estrogen-containing medicines, pregnancy, cortisone-type medicines and some epilepsy medicines can also affect the result, so they should be reported to your doctor.

Supplements containing biotin can make T4 appear falsely high with some immunological methods, creating a picture that looks like hyperthyroidism. If you take supplements, talk to your doctor about how long you need to pause them. If you have recently had a serious illness, it is important to mention it, because total T4 can change temporarily during this period. You do not need to go to a laboratory for the blood sample; the nurse can take the sample at your home or at your workplace.

What does the Total T4 result mean?

A high total T4 may indicate that the thyroid gland is overactive or that more thyroid medication than needed is being taken. However, in situations where carrier proteins are increased because of pregnancy, estrogen use or inherited causes, the value can be high even while the thyroid gland is working normally. In the early phase of thyroid inflammation, a temporary rise may also be seen as hormone stored in the gland is released into the blood. Measuring free T4 helps distinguish whether the elevation is caused by a true excess of hormone or by an increase in carrier proteins.

A low total T4 may suggest hypothyroidism, but it can also be seen in kidney diseases that cause protein loss in the urine, with the use of certain medicines, or during periods of serious illness. For this reason, the result is usually considered together with TSH and free T4. The range considered normal can vary by laboratory, age, sex and pregnancy status; you can see the range of the laboratory that runs the test on this page. Your doctor, who reviews your complaints and other tests together, determines what the value means for you.