PTH-Independent Hypercalcemia in Undiagnosed Adrenal Insufficiency

In contrast to PTH-dependent hypercalcemia, the differential diagnosis of PTH-independent hypercalcemia is extensive. Thorough history and physical examination can help direct the clinician in the right direction and avoid extensive and unnecessary work up. A 27-year-old-female with a medical history significant only for untreated subclinical hypothyroidism was admitted to the hospital for hypercalcemia and acute kidney injury after presenting with dizziness, nausea, vomiting, dyspnea, and multiple syncopal episodes prior to admission. Serum calcium on presentation was elevated to 15.9 (8.5–10.2 mg/dL) with an intact parathyroid hormone level of 3.6 (8.5–75 pg/mL). On initial presentation, she was hypotensive, tachycardic, and was noted to be underweight with a body mass index of 17 kg/m2. The patient’s skin was diffusely hyperpigmented, with increased pigmentation in the creases of her hands and on the sides of her fingers. Further lab evaluation was remarkable for hyponatremia and hyperkalemia as well as an undetectable 8 AM cortisol of <1.0 (7–25 µg/dL), adrenocorticotropic hormone (ACTH) level significantly elevated to >1,250 (6–58 pg/mL), aldosterone <4 (<21 ng/dL), and plasma renin activity elevated to 18 (<0.6–3 ng/mL/h). Antibodies to 21-alpha hydroxylase were positive, confirming a diagnosis of Addison’s disease. Prior to the workup confirming an elevated ACTH and low cortisol levels, the patient was treated with aggressive intravenous fluid repletion and calcitonin with overnight improvement in calcium to 11.4 mg/dL. After the diagnosis of primary adrenal insufficiency was confirmed, she was treated with stress doses of intravenous hydrocortisone then gradually tapered to physiologic doses of oral hydrocortisone and fludrocortisone with resolution of her hypercalcemia by the fourth day of hospitalization. Adrenal insufficiency is a known but uncommon cause of PTH-independent hypercalcemia, but the exact mechanism is unknown. Hypercalcemia is thought to result from a combination of a hypovolemic state seen in adrenal insufficiency which leads to decreased urinary calcium excretion as well as increased bone resorption, which may result from increased serum sclerostin concentrations. Adrenal insufficiency should be considered in the differential of PTH-independent hypercalcemia. This case highlights the improvement in hypercalcemia that is seen with correction of glucocorticoid deficiency, and supports delaying additional work up for other PTH-independent causes until appropriate treatment has been given.