Abstract Introduction Ectopic ACTH syndrome constitutes 18% of Cushing's syndrome cases, and in rare cases, ACTH secretion from medullary thyroid cancer cells is the source of ectopic ACTH production. Here we report a catastrophic Cushing's syndrome case owing to medullary thyroid cancer in which the source of ACTH production was demonstrated by lymph node ACTH washout. Clinical Case A 23 year-old male was admitted to other clinics with complaints of inability to lose weight despite diet and exercise, and weight gain of approximately 40 kg, which had been going on for the last two years. At his application, his weight was 160 kg and his body mass index was 42.51 kg/m2. On physical examination, clinical stigmata suggesting Cushing's syndrome such as moon face, buffalo hump, supraclavicular fat deposition, acanthosis nigricans, centripedal obesity, purple striae on the abdomen were noted. The biochemical and hormonal evaluation confirmed ACTH-dependent Cushing's syndrome (Table 1). To establish the source of excess ACTH, first, a pituitary MRI was performed which revealed a 4×2 mm adenoma on the left side of the gland. In PET-CT, a 24×20 mm hypoechoic nodule in the inferior of the right thyroid lobe, and several pathological lymph nodes at the right side of the neck were detected. Serum calcitonin levels were high (6528 ng/L) and fine-needle aspiration biopsy from the thyroid nodule documented medullary thyroid cancer. The patient underwent bilateral total thyroidectomy and right cervical lymph node dissection. The pathology report was compatible with metastatic medullary thyroid cancer. After surgery, the remission of hypercortisolemia was not achieved and the patient was started on metyrapone treatment before the permanent treatment plan. Postoperative Gallium-68 DOTA PET-CT indicated residual disease and increased uptake in retropharyngeal, bilateral cervical level 2-4, right level 3, bilateral central, and bilateral lower cervical lymph nodes. The persistence of hypercortisolemia after surgery was most likely attributed to residual disease, however, it was considered that pituitary adenoma may also be the cause of ACTH secretion. Therefore, to confirm the source of ACTH secretion, ACTH washout samples were taken from the cervical lymph nodes and were found to be high as 958 ng/L. Reoperation was performed for recurrent mass and lymph node metastases. Conclusion Medullary thyroid cancer should definitely be considered in cases of ectopic ACTH syndrome. ACTH washing from the suspected lymph node may be an option for diagnostic purposes. In addition to the patient's primary tumor, hypercortisolism should also be treated.Table 1.Cushing's assessment tests
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