Endocrine Abstracts

Hyponatraemia is a very common electrolyte abnormality with varied presenting features depending on the underlying cause. The authors report the case of a 75 years old, previously fit, gentleman who presented with weight loss, lethargy, and blackouts. He was admitted to the hospital repeatedly under the general physicians over an 8 month period.

Investigations revealed persistent hyponatraemia (serum sodium 113–120 mmol/l, serum osmolality 258 mOsm/kg, urine osmolality 393 mOsm/kg, urinary sodium 40 mmol/l) consistent with a diagnosis of syndrome of inappropriate antidiuretic hormone secretion (SIADH), macrocytic anaemia and partial hypopituitarism (with secondary hypothyroidism, low testosterone and gonadotrophins and a reduced GH response as demonstrate by a diminished glucagon stimulation response). Cortisol response was normal. CT scan thorax, abdomen, and pelvis did not demonstrate any underlying cause and MRI pituitary was normal. He was commenced on thyroid, testosterone and GH replacement by the general medical team. This was rationalised to levothyroxine replacement by the endocrine team. All other investigations that were performed failed to identify the underlying cause. Despite best management of his hyponatraemia his condition continued to worsen and he subsequently died two weeks later. A post-mortem was then performed and a diagnosis of intravascular large B-cell lymphoma was made with intravascular infiltration of the pituitary gland.

The authors recommend that endocrinologists should be involved at the outset in the management of patients with persistent hyponatraemia. Intravascular large B-cell lymphoma should be considered as a rare differential diagnosis of hyponatraemia. A literature review identified a similar case where rituximab was used in the treatment of large B-cell lymphoma resulting in a gradual resolution of hypopituitarism. This highlights the potential benefit of early diagnosis and management in the setting of SIADH associated with malignancy.