A 36-year-old woman with postpartum hypopituitarism (Sheehan's syndrome: SS) developed postpartum autoimmune thyroiditis (PPAT). She delivered a baby by Caesarean section (620 mL blood loss). At 1 month post partum, she developed thyrotoxicosis due to painless thyroiditis (autoimmune destructive thyroiditis). She was positive for antithyroid antibodies. Postpartum and hypoadrenalism-induced exacerbation of autoimmune thyroiditis caused the thyrotoxicosis due to autoimmune destructive thyroiditis. ACTH was undetectable. She had ACTH deficiency and secondary hypoadrenalism. Hydrocortisone was started. At 6 months post partum, she was referred to us with hypothyroidism. Thyroxine was administered. She had thyrotoxicosis at 1-2 months post partum and then hypothyroidism. She was diagnosed with PPAT. She had hypopituitarism, ACTH deficiency (secondary hypoadrenalism), low prolactin with agalactia, and low LH with failure to resume regular menses. She had empty sella on MRI. She was diagnosed with SS. Three cases with SS have been reported to develop PPAT. Postpartum immunological rebounds and hypoadrenalism-induced immunological alterations (or a combination of the two) might have been responsible for the PPAT.