Thyroid crisis following painless thyroiditis initially diagnosed as syndrome of inappropriate antidiuresis

Painless thyroiditis is a destructive disorder associated with immune dysregulation. Although an abrupt reduction in corticosteroid activity has been implicated, the precipitating role of endogenous adrenal insufficiency remains unclear. A 66-year-old woman was admitted to a referring hospital with anorexia, vomiting, and hyponatremia and was diagnosed with syndrome of inappropriate antidiuresis (SIAD). Despite correction of serum sodium levels, anorexia persisted. Two months later, she developed fever, tachycardia, impaired consciousness, and severe hypercalcemia and was transferred to our institution. Laboratory evaluation revealed marked thyrotoxicosis with suppressed thyroid-stimulating hormone and elevated free thyroxine levels, along with inappropriately low adrenocorticotropic hormone (ACTH) and cortisol levels. The patient met the diagnostic criteria for thyroid crisis. Thyroid antibody testing and imaging findings were consistent with painless thyroiditis. Secondary adrenal insufficiency was confirmed by a blunted cortisol response to a rapid ACTH stimulation test. Treatment with intravenous hydrocortisone and empiric potassium iodide resulted in rapid clinical improvement. This case suggests an association between unrecognized secondary adrenal insufficiency initially diagnosed as SIAD and the subsequent occurrence of painless thyroiditis and thyroid crisis. Evaluation of adrenal function should be considered in patients with hyponatremia and thyrotoxicosis.