Thyroxine or T4 is a hormone produced by the thyroid gland. It is through T4 that the body control rate is measured. It is all regulated in a feedback system. The doctor may order the T4 test if the doctor sees the abnormal result on a TSH test or suspects of hyperthyroidism or hypothyroidism. If the patient shows increased high rates, anxiety, sudden weight loss or being the insomnia, tremors in the hands or have visual disturbances, he can be a suspect for hyperthyroidism, and the T4 test can be called in for. Whereas weight gains, dry skin, cold intolerance, fatigue, hair loss, irregularity in menstruation cycle of women, constipation can be symptoms of hypothyroidism.