FRI355 Hypokalemia-induced Nephrogenic Diabetes Insipidus In A Patient With Medullary Thyroid Carcinoma And Paraneoplastic Cushing’s Syndrome

Disclosure: S. Jhawar: None. S. Jumani: None. Y. Sterrett: None. H. Elenius: None. P. Veeraraghavan: None. C. Ryan: None. J. Del Rivero: None. S.M. Sadowski: None. N. Nilubol: None. J.G. Verbalis: None. L.K. Nieman: None. E. Globa: None. M.S. Zemskova: None. J. Klubo-Gwiezdzinska: None. S. Gubbi: None. Introduction: Nephrogenic diabetes insipidus (NDI) is commonly associated with the use of medications such as lithium, antibiotics, and chemotherapeutic agents or with renal disorders. Rarely, NDI can be a manifestation of paraneoplastic conditions. We describe a case of NDI caused by severe hypokalemia in a patient with metastatic medullary thyroid cancer (MTC) and paraneoplastic Cushing’s syndrome (CS). Clinical Case: A 47-year-old male with sporadic, metastatic MTC (RET M918T pathogenic variant) was referred to our institution for further management. The patient had florid metastases to the liver, lungs, and skeleton, with a calcitonin of 158,535 pg/mL [normal (NL): <14.3], and a CEA of 3,634 pg/mL (NL: 0.8-3.4), but surprisingly, without an intrathyroidal primary tumor. Vandetanib and octreotide were previously trialed but discontinued due to financial constraints. The patient had diarrhea, but no other symptoms. Physical exam revealed cervical lymphadenopathy, facial plethora, dorsocervical fat pad, and a single purple stria on the chest. Further work up revealed markedly elevated 24-hr urine free cortisol (12,460 mcg/24hr; NL: 35-45), ACTH 116 pg/mL (normal 5.0-46.0), and an undetectable serum aldosterone and plasma renin activity, raising suspicion for paraneoplastic ectopic-CS. Immunostaining of the MTC biopsy specimen showed ACTH positivity. During the inpatient admission, polyuria of >3L/day was noted on two consecutive 24-hour urine measurements. Laboratory investigations revealed hypernatremia (serum sodium: 147 mmol/L; NL: 136-145), elevated serum osmolality (303 mOsm/Kg; NL: 278-298), and a low urine osmolality (215 mOsm/kg; NL: 300-900), thus establishing the diagnosis of DI. A simultaneous, elevated plasma copeptin (36.3 pmol/L; NL: <13.1) further suggested NDI. Also noted was severe hypokalemia (serum potassium: 1.7 mmol/L; NL: 3.5-5.1), with normal serum magnesium and calcium. A thorough review of patient’s medication list did not reveal any culprit drug for hypokalemia or NDI. Hypokalemia was managed with aggressive oral and intravenous potassium repletion. A bilateral adrenalectomy was performed to treat the ectopic-CS, which improved the hypokalemia and resolved the NDI. Three months later, the patient unfortunately died due to progression of MTC. Conclusion: Severe hypokalemia causes autophagic degradation of aquaporin-2 channels in the renal tubules and is an under-recognized cause of NDI. Profound hypercortisolism in ectopic-CS may result in cortisol-mediated overactivation of mineralocorticoid receptors that can cause renal potassium loss leading to NDI. Only 0.6% of MTCs are associated with ectopic-CS (PMID: 16029131). Our report highlights hypokalemia-induced NDI as a manifestation of paraneoplastic CS resulting from MTC. Presentation: Friday, June 16, 2023