This test is vital to detect thyroid disorders and monitor the patient during the treatment of the hypo or hyperthyroidism. T4 test measures the level of the (T4- Thyroxine) hormone in the blood, which is one of the hormones that produced by the thyroid gland. The thyroid gland, located beneath the larynx, is the internal thermostat of the body that regulates temperature by secreting T4- Thyroxine and T3- Triiodothyronine hormones. When the activity of the thyroid gland is reduced, the levels of the hormones (T4- Thyroxine / T3- Triiodothyronine) are decreased. The levels of the hormones (T4- Thyroxine / T3- Triiodothyronine) correlate inversely with TSH and both are required to determine accurately the functionality of the thyroid gland.