T4 (thyroxine) is the main hormone produced by the thyroid gland. A laboratory test can be done to measure the amount of free T4 in your blood. Free T4 is the thyroxine that is not attached to a protein in the blood.
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 biotin.
Pregnancy and some diseases, including kidney and liver disease, can also affect the results of this test.
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.