What is the thyroxine (T4) test?
Thyroxine (T4) is a hormone produced by the thyroid gland. A test of the amount of T4 in the blood is a way to see how well the thyroid gland is working. If you have to take thyroid hormone (often called thyroid replacement hormone), this test also makes sure you have the correct amount in your blood. The thyroid gland is located at the lower front of the neck. Its main job is to make T4 and release it into the bloodstream. T4 circulates throughout the body, affecting all your organs. T4 regulates metabolism, like a thermostat regulates a furnace or air conditioner. The amount of T4 produced and released by the thyroid gland is controlled by the pituitary gland, which is located in the brain. The test for T4 is one of several tests that can be done to check the functioning of the thyroid gland.
