A 70-year-old woman with Cogan's syndrome first presented with central diabetes insipidus and then developed secondary hypothyroidism. Magnetic resonance imaging revealed a diffuse pituitary swelling without evidence of tumor. High-dose glucocorticoid therapy administered to treat Cogan's syndrome was very effective in suppressing the inflammatory process, and resulted in the reversal of the pituitary swelling and partial recovery of thyroid stimulating hormone secretion. This is the first case of hypopituitarism associated with Cogan's syndrome, a form of autoimmune vasculitis. The glucocorticoid-responsive pituitary lesion is best explained by autoimmune hypophysitis which shows pituitary swelling and is known to often associate with other autoimmune phenomena.