Of the various hypothalamic-pituitary-end organ axes, the thyroid and adrenal systems have been implicated most often in affective disorders. Patients with primary thyroid disease have high rates of depression, and patients with Addison's disease or Cushing's syndrome have relatively high rates of affective and anxiety symptoms. However, the major support for these endocrine axes in the pathophysiology of mood disorders comes from studies in which alterations in components of the hypothalamic-pituitary-thyroid (HPT) and the hypothalamic-pituitary-adrenal (HPA) axes have been documented in patients with primary depression. Concerning the HPT axis, depressed patients have been reported to have: (a) alterations in thyroid-stimulating hormone response to thyrotropin-releasing hormone (TRH); (b) an abnormally high rate of antithyroid antibodies; and (c) elevated cerebrospinal fluid (CSF) TRH concentrations. Moreover, tri-iodothyronine has been shown conclusively to augment the efficacy of various antidepressants. Concerning the HPA axis, depressed patients have been reported to exhibit: (a) adrenocorticoid hypersecretion; (b) enlarged pituitary and adrenal gland size; and (c) elevated CSF corticotropin-releasing factor concentrations. All of the HPA axis alterations in depression studied thus far are state-dependent, whereas the HPT axis alterations may be partially trait and partially state markers.