Thyroxine (T4), Total LabCorp

The thyroid gland produces and secretes T4, otherwise known as thyroxine. Proteins bind to T4 and carry it throughout the bloodstream. Once in he 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 hormone production. This test measures the total T4 and it is recommended to also add free T4 for a more comprehensive thyroid analysis. A complete thyroid panel will provide a full review of thyroid hormone function. 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, hair loss, constipation and cold intolerance. Fasting is not required for this test. It is recommended to take all medications as prescribed and not to take any supplements on the same day of the test.