Late-onset Schmidt's Syndrome Presenting with Severe Hyponatremia: A Case Report

Schmidt's syndrome (SS) is a subtype of polyglandular autoimmune syndrome type-2 combining autoimmune thyroiditis (AIT) and autoimmune Addison's disease (aAD). It occurs most frequently in young adult females, and aAD is the most common initial manifestation [1]. We present a rare case of SS with late-onset aAD and severe hyponatremia as the first sign.

A 73-year-old woman presented to the emergency department (ED) with a 10-day history of vomiting, diarrhea, and altered mental status. Her past medical history was remarkable for AIT and hypokinetic cardiomyopathy. Moreover, she had recently undergone a 2-week course of corticosteroid therapy for vertiginous symptoms, reporting subjective well-being. In ED, she appeared confused and hypotensive. Blood tests revealed a sodium level of 99 mEq/l with normal potassium. Initial treatment with saline infusions were started, followed by ex juvantibus intravenous hydrocortisone awaiting hormone results, which proved consistent with primary adrenal insufficiency (ACTH 1314 pg/ml, cortisol 4.72 ug/dL). Replacement therapy with both hydrocortisone and fludrocortisone was then implemented, with substantial clinical improvement and normalization of sodium levels. However, the patient later developed right heart failure and hypokalemia, which were likely caused by overreplacement and resolved after adjusting the treatment regimen. The final diagnosis of aAD was confirmed by positive adrenal autoantibodies.

aAD should be suspected in each case of severe hyponatremia [2], especially in patients with AIT independent of age. Furthermore, caution is needed in managing high-dose glucocorticoids along with fludrocortisone in elderly patients with cardiac disease to limit the risk of excessive mineralocorticoid activity and heart failure [3].