Free T4 test

A blood sample is needed.

Your health care provider will tell you if you need to stop taking any medicines that may affect the test result. In general, test results are not affected by other medicines you may be taking. However, certain supplements including biotin (vitamin B7) can affect the results. Tell your provider if you are taking…

When the needle is inserted to draw blood, some people feel moderate pain. Others feel only a prick or stinging. Afterward, there may be some throbbing or a slight bruise. This soon goes away.

Your provider may recommend this test if you have signs of a thyroid disorder, including: Abnormal findings of other thyroid blood tests, such as TSH or T3 Symptoms of an overactive thyroid Symptoms of an underactive thyroid Hypopituitarism -- when the pituitary gland does not produce enough of its hormones, including…

A typical normal range is 0.8 to 1.9 nanograms per deciliter (ng/dL), or 10.3 to 24.5 picomoles per liter (pmol/L). Normal value ranges may vary slightly among different laboratories. Some laboratories use different measurements or may test different specimens. Talk to your provider about the meaning of your specific…

To fully understand results of the free T4 test, results of other thyroid blood tests, such as TSH or T3, may be needed. Test results may also be affected by pregnancy, estrogen level, liver problems, more severe body-wide illnesses, and inherited changes in a protein that binds T4. A higher than normal level of T4…

There is little risk involved with having your blood taken. Veins and arteries vary in size from one person to another and from one side of the body to the other. Obtaining a blood sample from some people may be more difficult than from others. Other risks associated with having blood drawn are slight, but may…

Faix JD. Thyroid function testing (thyrotropin, triiodothyronine, and thyroxine). In: Robertson RP, ed. DeGroot's Endocrinology . 8th ed. Philadelphia, PA: Elsevier; 2023:chap 68. Guber HA, Oprea M, Russell YX. Evaluation of endocrine function. In: McPherson RA, Pincus MR, eds. Henry's Clinical Diagnosis and…