Medullary Thyroid Carcinoma Presenting with Severe Hypokalemia

Introduction: Medullary thyroid carcinoma (MTC) accounts for about 1-2% of all thyroid malignancies. 70 % of most medullary thyroid carcinomas are sporadic in nature. The typical age at initial presentation ranges between 41 and 55 years. Approximately 0.6% of cases of MTC have ectopic production of ACTH. The objective of this case is to bring attention to the association between ectopic ACTH or CRH causing symptoms in the setting of medullary thyroid carcinoma Case Description: Thirty-eight-year-old male, presented to the hospital due to left upper extremity swelling, redness, and pain. He was diagnosed with acute cellulitis and started on appropriate antibiotics. At the same time, aggressive replacement of potassium resulted in only minimal improvement and recurrence of severe hypokalemia prompting further workup. Urine electrolyte panel results were suggestive of renal potassium wasting defect. The aldosterone level was normal and direct renin level was normal The random cortisol level was increased. Dexamethasone suppression test was done which showed elevated cortisol levels confirming ACTH production. Brain MRI revealed no pituitary lesions. Due to a lack of improvement in the left arm swelling, the patient underwent a CT scan with IV contrast to look for possible obstruction which showed a mass in the thyroid region with abdominal lymph nodes, and multiple hepatic lesions. A liver biopsy was done which was consistent with medullary thyroid carcinoma. The patient was started on metyrapone with an improvement in cortisol levels. The patient was then started on Selpercatinib 160 mg twice per day, with regular follow-ups with heme/onc, nephrology, and endocrinology. His potassium and glucose were well controlled. However, he developed a recurrence of cancer. Discussion: The diagnosis of ectopic ACTH production secondary to MTC is based on the presence of hypercortisolism not suppressed by high cortisol, absence of pituitary adenoma, and Cushing syndrome symptoms. MTC can spread to lymph nodes, and the lungs, liver, and brain via blood. Treatment for ACTH production includes adrenolytic agents or suppression of steroidogenesis. In extreme cases, bilateral adrenalectomy may be needed. Tyrosine kinase inhibitors including selpercatinib, sorafenib, vandetanib, and cabozantinib are used for chemotherapy for MTC. Due to the high resistance rates of MTC to tyrosine kinase inhibitors, screening for recurrence is essential.