Ectopic Cushing’s Syndrome and Severe Hypocalcemia Due to Medullary Thyroid Cancer Responsive to Selpercatinib

Introduction: Cushing’s syndrome (CS) due to ectopic ACTH production from medullary thyroid carcinoma (MTC) is characterized by rapid progression of disease, leading to hyperglycemia and hypokalemia. However, hypercortisolemia leading to hypocalcemia is rarely seen. Initiation of selpercatinib greatly improved hypocalcemia and ectopic CS in this case. Clinical Case: A 41-year-old man with a history of MTC (variant RET p.M918T) post thyroidectomy in 2018 developed progressive weight gain, lower extremity edema, weakness, new onset diabetes, severe refractory hypocalcemia and hypokalemia requiring multiple hospitalizations. On initial presentation to our institution, he was lethargic, had multiple ecchymoses, peripheral edema and proximal myopathy. Laboratory evaluation revealed Ca 5.4 mg/dL (NR 8.9 - 10.3), albumin 3 g/dL (NR 3.5 - 5.1), iPTH 1.4 pmol/L (NR 1.6 - 6.9), 25-OH vitamin D 23 pg/mL (NR 25–80) while taking elemental calcium 1500 mg every 6h, calcitriol 0.25 mcg/d and vitamin D3 1000 IU/d. Serum cortisol measured at 9:30 pm was 136 ug/dL (NR 2.5–11.9), ACTH 1,145 pg/mL (NR 7.2–63.3) and 24-h UFC 27,629 ug/d consistent with CS due to ectopic ACTH production. Calcitonin and CEA were 18,687 pg/mL (NR 0–7.5) and 3,766 ng/mL (NR 0–4.7). CT abdomen revealed numerous bilateral liver lesions and bilateral adrenal hyperplasia. In addition to high doses of oral calcium and calcitriol, he required calcium drip up to 1.5mg/kg/hr for about 1 week. He simultaneously began cabozantinib, ketoconazole and metyrapone. Hospital course was complicated by infections and recurrent scrotal bleeding, so he was switched to selpercatinib. Two days after starting selpercatinib, ketoconazole was discontinued, and metyrapone has been gradually reduced. Most recent calcitonin was 149 pg/mL, CEA 97.8 ng/mL and 24-h UFC 10 ug/d on metyrapone 250 mg twice daily. Similarly, refractory hypocalcemia greatly improved, last serum Ca was 8.3 mg/dL on elemental calcium 480 mg/d. He has made significant clinical gains and has returned home from rehab. Clinical Lesson: Hypocalcemia is rarely described as a complication in patients with CS. Our patient had underlying hypoparathyroidism and vitamin D deficiency; however, hypocalcemia was initially refractory to high doses of calcium and calcitriol and only improved with treatment of CS. We suspect hypercortisolemia impaired 25 to 1,25 D activation, thereby reducing calcium absorption, and likely inducing hypercalciuria. These deleterious effects of severe hypercortisolemia combined with underlying hypoparathyroidism led to severe and refractory hypocalcemia requiring repeated admissions, and only improved once his ectopic CS due to MTC was recognized and controlled. The RET kinase inhibitor, selpercatinib, induced a rapid decline in calcitonin, CEA and ACTH levels, and with metyrapone, enabled control of hypercortisolemia and its complications.