For centuries there has been convincing evidence that diseases of the thyroid gland may produce psychiatric symptoms. Nowadays systematic data are available concerning a higher incidence of thyroid diseases in psychiatric patients and vice versa a higher incidence of psychiatric disorders in thyroid patients. A more subtle approach concerns challenge tests like the TRH test. It could be shown that depressive patients show a blunted TSH response to TRH in 30-40% of the cases. This might lead to a definition of a subgroup of depressives from a psychoneuroendocrinological point of view. But it also might have an impact on the prediction of treatment outcome in psychopharmacological treatment approaches.