A 53-year-old Japanese man was admitted with a 3-month history of transient headache followed by general fatigue and weight loss. He had a history of ocular myasthenia gravis which had been in remission following thymectomy 30 years ago. He had a small diffuse goiter without tenderness, and was diagnosed as having painless thyroiditis with mild thyrotoxicosis on admission. Endocrinological studies showed he had isolated adrenocorticotropin deficiency. Magnetic resonance imaging of the pituitary gland revealed no abnormalities. His symptoms improved soon after replacement of glucocorticoid. After an episode of hypothyroidism, he spontaneously became euthyroid. It is likely that thyrotoxicosis uncovered adrenal insufficiency that had developed insidiously, and hypoadrenocorticism-induced immunological changes may have triggered the development of painless thyroiditis. Moreover, thymectomy might have facilitated the development of pituitary and thyroid autoimmunity.