Since the introduction of radionuclides into clinical medicine, a number of specific tests have been designed to test thyroid function and to diagnose disease of the thyroid gland. These studies can be broadly grouped into in vivo and in vitro studies. Using 131I and 125I , tests have been designed to measure thyroid function at the hypothalamic, pituitary, or thyroid level and at the peripheral level by radioimmunoassay or radioassay of circulating thyroid hormones. The following schema of testing may be used to assess thyroid function