Thyroxine, otherwise known as T4, is a hormone produced and secreted by the thyroid gland. At the tissue level, T4 is converted into the more active form of the hormone called T3. For this reason, T4 is considered a measurement of total production of thyroid hormone. High levels of 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 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. Take all medications as prescribed.