Thyroid function sets our metabolism, controls our energy and influences our mood. We often feel tired, depressed and gain weight when our thyroid is slow. We can have anxiety and rapid heart beat when our thyroid works to fast. The thyroid is controlled by the pituitary gland in the brain. This system is controlled by a feedback mechanism of TSH (thyroid stimulating hormone from the brain) and T4(the storage form of the thyroid hormone.) We measure these two hormones to make sure the thyroid is functioning correctly. T4 is also converted to T3 (the active form of the thyroid hormone) by enzymes in the liver and elsewhere in the body. We also check T3 to make sure the whole system is functioning properly and we are working at our optimal thyroid function.