Metastatic medullary thyroid carcinoma presenting as ectopic Cushing's syndrome

A 41-year-old male presented to the Emergency Department with a 6-month history of back and hip pain. Skeletal survey revealed bilateral pubic rami fractures and MRI of the spine demonstrated multiple thoracic and lumbar fractures. Secondary work up for osteoporosis was undertaken. There was no evidence of hyperparathyroidism and the patient was vitamin D replete. Testosterone (T) was low at 1.7 nmol/L (8.6-29.0) and gonadotrophins were undetectable. The patient failed a 1 mg dexamethasone suppression test (DST) with a morning cortisol of 570 nmol/L (<50) and subsequently a low dose DST with a cortisol post 48 h of dexamethasone of 773 nmol/L (<50) and an elevated ACTH 98 ng/L. A corticotropin-releasing factor (CRF) test suggested ectopic ACTH secretion. The patient was commenced on teriparatide for osteoporosis and metyrapone to control the hypercortisolaemia. A positron emission tomography (PET) scan to look for the source of ACTH secretion demonstrated right neck adenopathy. Biopsy and subsequent lymph node dissection were performed and histology revealed a metastatic neuroendocrine tumour. Immunostaining was positive for calcitonin and thyroid transcription factor 1 (TTF1). Serum calcitonin was also significantly elevated at 45 264 ng/L (<10). The patient proceeded to a total thyroidectomy and left neck dissection. Histology confirmed a 7 mm medullary thyroid carcinoma (MTC). Post-operatively, the patient commenced vandetanib therapy and achieved a clinical and biochemical response. After approximately 18 months of vandetanib therapy, the patient developed recurrent disease in his neck. He is currently on LOXO-292 and is doing well 36 months post-diagnosis.

Unexplained osteoporosis requires thorough investigation and the workup for secondary causes is not complete without excluding glucocorticoid excess. MTC should be considered when searching for sources of ectopic ACTH secretion. Resistance to tyrosine kinase inhibitors is well described with MTC and clinicians should have a low threshold for screening for recurrent disease.