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 hormone production. This test measures the total T4 and it is recommended to also add free T4 or for a more comprehensive thyroid analysis order the Complete Thyroid Profile which provides a full review of thyroid hormone function. High 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. Take all medications as prescribed. Do not take supplements the day of the test. This test is also included in the Complete Thyroid Profile.