The occurrence of mental changes in patients with hyperthyroidism and hypothyroidism is well known. In contrast, the role of thyroid function in patients with depression is still controversial. However, it is certain that some depressed patients have a tendency towards hypothyroidism.Thyroid function should be carefully checked in depressed patients, especially in those who are elderly or in the postpartum period. The thyrotrophin-releasing hormone (TRH; protirelin) test is a useful psychoendocrine test.Thyroid hormones augment the effect of tricyclic antidepressants, and large dosages of these hormones influence the periodic recurrence of mood disorders. Therefore, thyroid hormones can be used as adjunctive therapy in patients with refractory depression. They may also be considered as candidate therapy for the treatment of intractable cycling psychoses.