Free T4 (FT4)

What is free T4?

T4 (thyroxine) is the hormone the thyroid gland releases into the blood in the largest amounts, and it sets the body's metabolic rate. In the tissues, it is converted as needed into the more active T3. The vast majority of T4 in the blood is bound to carrier proteins; only the small free portion can enter cells and take effect. This hormone affects many functions, from heart rate to body temperature and from digestion to mood.

The free T4 test measures this free portion using an immunoassay and, together with TSH, forms the basis of thyroid evaluation. TSH shows the signal coming from the pituitary, while free T4 shows the hormone the gland actually produces and that can reach the tissues. Because it is much less affected than total T4 by changes in the amount of carrier protein, it is generally preferred over total T4 in everyday practice.

When is a free T4 test ordered?

Free T4 is mostly ordered when TSH is abnormal, to understand how pronounced the problem is. If TSH is high but free T4 is normal, the condition is considered subclinical (mild) hypothyroidism; if free T4 is also low, it is considered overt hypothyroidism. Because TSH can be misleading in pituitary gland diseases and in the first months of hyperthyroidism treatment, free T4 is the guiding test during these periods.

• High or low TSH
• Fatigue, weight change, feeling cold or heat intolerance
• Dose monitoring during treatment with thyroid hormone medication
• Monitoring in the early phase of hyperthyroidism treatment
• Evaluation in pituitary gland disease or after pituitary surgery
• Monitoring thyroid function during pregnancy

How do I prepare for a free T4 test?

No fasting is required for this test. Free T4 can rise temporarily in the few hours after you take thyroid hormone medication; for this reason, unless your doctor tells you otherwise, you are advised to take your medication after giving blood. Having follow-up measurements done at similar times of day makes it easier to compare results.

Hair, skin and nail supplements containing biotin (vitamin B7) can make free T4 falsely high with some methods, giving the impression that the thyroid gland is overactive. If you use such a product, you may need to pause it before the test after consulting your doctor. Some heart rhythm medications and epilepsy medications can also change free T4 levels; tell your doctor about the medications you take. If you wish, a nurse can come to your home or workplace to take your blood sample.

What does a free T4 result mean?

High free T4 together with low TSH suggests that the thyroid gland is overactive (hyperthyroidism); Graves' disease, toxic nodules, thyroid inflammation or taking too much medication are among the possible causes. Low free T4 together with high TSH indicates that the gland is underactive (hypothyroidism), and the most common cause is Hashimoto's thyroiditis. In people taking thyroid hormone medication, free T4 close to the upper limit of the range may be expected, depending on when the medication was taken.

When free T4 is low and TSH is also low or normal, this may suggest that the problem lies in the pituitary gland, or it may reflect the temporary effect of a severe illness. If the results do not fit with TSH, your doctor may have the test repeated, taking into account possible interferences with the measurement. The reference range varies from laboratory to laboratory, as well as by age, sex and pregnancy status; the range shown on this page is the laboratory's own range. The final interpretation should be made by your doctor, taking your symptoms and your other thyroid tests into account together.