• His serum osmolality was raised (297mOsm/kg) with an inappropriately low urine osmolality (143mOsm/kg). • He had a low serum testosterone level (5.9nmol/L) in the presence of inappropriately normal Luteinising Hormone and Follicle Stimulating Hormone levels, suggesting hypogonadotrophic hypogonadism. • His prolactin, thyroid and adrenal function tests were normal. • An MRI scan revealed a large thin-walled complex cyst arising from his hypothalamus (Figure 1, arrows). • CT (chest, abdomen, pelvis) was normal. Table 1-Results of Water Deprivation Test We present a combination of cranial DI and hypogonadotrophic hypogonadism secondary to a possible craniopharygioma. Other causes such as a meningioma, germ cell tumour, epidermoid and dermoid cyst, hypothalamic hamartoma, arachnoid cyst, Rathke cleft cyst, carotid aneurysm, and cavernous haemangioma all make interesting differential diagnoses. • He was started on Desmopressin with appreciable symptomatic improvement.