When Nephrology and Endocrine Axes Meet: Hyponatremia and Hyperkalemia Due to Late-Onset Autoimmune Polyglandular Syndrome Type 2

Introduction: Polyglandular autoimmune syndrome type 2 (PAS-2) is characterized by autoimmune adrenal insufficiency coupled with autoimmune thyroid disease and/or type 1 diabetes. It is more common in women, typically presenting in the third or fourth decade of life. We present an atypical case of an elderly woman presenting with confusion and several electrolyte abnormalities found to have adrenal insufficiency (AI) due to late-onset PAS-2. Case Description: A 73-year-old female with Hashimoto’s thyroiditis for 21 years (+thyroid peroxidase antibodies), type 2 diabetes for 2 years, and osteoporosis was hospitalized with 1 day of confusion. She was found to have a sodium of 126 mEq/L, potassium of 5.7 mEq/L, bicarbonate of 19 mEq/L and anion gap of 13. Labs 4 years prior showed chronic hyponatremia (Na 130-136 mEq/L), hyperkalemia (K 5-5.4 mEq/L), and metabolic acidosis (HCO3- 19-22 mEq/L). Systolic blood pressure ranged from 80–110 mmHg. Relevant workup included plasma aldosterone <1 ng/dL, plasma renin activity of 98 ng/mL/hr and a urine potassium <5 mEq/L. Morning cortisol level was 7.7 mcg/dL. Due to suspected adrenal insufficiency, an ACTH level was sent and was markedly elevated at 668 pg/mL. Serum cortisol did not rise upon stimulation with cosyntropin. She was subsequently started on fludrocortisone with resolution of symptoms and electrolyte abnormalities (Figure). Autoantibodies against 21-hydroxylase were positive, consistent with primary AI. Together with her history of autoimmune thyroid disease, a diagnosis of PAS-2 was made. Discussion: PAS-2 is a rare autoimmune condition caused by lymphocytic infiltration of endocrine organs. While it typically presents in early to mid-adulthood, this case demonstrates that PAS-2 can develop insidiously in older adults. Altered sodium and potassium handling and impaired H+ secretion were consequences of mineralocorticoid deficiency due to unrecognized AI. This case illustrates how AI alters distal tubular function, leading to hyponatremia, hyperkalemia, and metabolic acidosis.