Knowledge about the physiology of the endocrine glands has been expanded in the past several years. It now appears that the secretion of the various pituitary hormones is conditioned by specific releasing hormones secreted in the hypothalamus. Thyrotropin-releasing hormone conditions the pituitary to release thyroid-stimulating hormone when the serum thyroid hormone concentration falls below a certain level. A radioimmunoassay for TSH helps the physician to distinguish the different etiologies of hypothyroidism, to confirm the diagnosis of "euthyroid" Graves' disease and to evaluate pituitary function.