Thyroxine (T4), Free, Direct (Free T4) LabCorp

The thyroid gland produces and secretes T4, otherwise known as thyroxine. Proteins bind to T4 and carry it throughout the bloodstream. Once in the tissues, T4 is released from the proteins and is now free to convert into the more active form called T3. For this reason, many doctors believe that measuring free T4 is a more sensitive test for thyroid production. Elevated levels of free T4 are associated with symptoms such as anxiety, insomnia, diarrhea, and muscle aches and pains. These symptoms collectively are known as hyperthyroidism. Low levels of free T4, known as hypothyroidism, are associated with symptoms such as depression, weight gain, dry skin, hair loss, constipation, and cold intolerance. Fasting is not required for this test. It is recommended to take all medications as prescribed and refrain from taking supplements on the same day as the test.