The syndrome of inappropriate antidiuretic hormone secretion (SIADH) is defined as hyponatremia with inappropriately concentrated urine in a euvolemic patient. SIADH is associated with a wide spectrum of clinical conditions. In the hospital, hyponatremia carries significant mortality with a prolonged duration of inpatient stay. It is imperative that the underlying cause is appropriately investigated and such patients are closely monitored. This article presents a case of difficult-to-treat hyponatremia secondary to SIADH in a patient with a rare isolated central nervous system (CNS) relapse from a non-Hodgkin’s lymphoma (NHL). A relapse, particularly affecting the CNS, carries a poor prognosis. The patient was started on dexamethasone and offered treatment with methotrexate but declined. The hyponatremia failed to respond to fluid restriction and demeclocycline. The hyponatremia responded to a single dose of tolvaptan. Clinicians should have a low index of suspicion for a relapse of lymphoma as a cause of difficult to treat hyponatremia in any patient who has previously had remission from lymphoma treatment.

• 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.