Medullary Thyroid Carcinoma Presenting with Severe Hypokalemia

Several case reports have been reported of ectopic ACTH production secondary to MTC. 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. This case is about a 38-year-old male, with recently diagnosed hypertension, who presented to the hospital due to left upper extremity swelling and significant hypokalemia who was found to have Cushing's syndrome and later metastatic medullary thyroid carcinoma. He was started on metyrapone with an improvement in cortisol levels. The patient was then started on Selpercatinib, with a good response. However, he developed a recurrence of cancer. 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.