Endocrine Abstracts

A 58-year-old lady presented to hospital with abdominal pain, nausea, and tiredness, and was referred to endocrinology with symptomatic hyponatraemia (serum Na⁺ 112 mmol/l). Six months previously her serum sodium was normal. She was euvolaemic, and adrenal insufficiency and thyroid dysfunction were excluded. Laboratory investigations were suggestive of syndrome of inappropriate antidiuretic hormone (SIADH) and no causative medications were identified. A CT of her thorax, abdomen, and pelvis revealed a right upper lobe lung nodule and cysts in the liver, kidneys, and adnexae. Her hyponatraemia was unresponsive to fluid restriction or demeclocycline. She was treated with Tolvaptan 7.5 mg on alternate days and subsequently her serum sodium stabilised (128–135 mmol/l). The cysts and right lung nodule were investigated extensively with further imaging studies and appropriate multidisciplinary input; all were felt to be benign.

She was readmitted to hospital three months later with oral ulcers, lethargy, and dyspnoea. Her sodium had normalised (137–140 mmol/l) but she had developed new-onset hypokalaemia (2.3–3.3 mmol/l) and pancytopenia. A further CT scan showed a left hilar lung mass with left upper lobe collapse and multiple spiculated nodules throughout the right upper lobe. It showed marked enlargement of both adrenal glands. Paired serum cortisol was 3107 nmol/l and ACTH was 561 ng/l. She was diagnosed with metastatic small cell lung cancer with paraneoplastic SIADH and Cushing’s syndrome, and unfortunately deteriorated rapidly.

This case demonstrates the need to be aware that multiple paraneoplastic syndromes can co-exist. Contrary to the more common sequence of presentation, Cushing’s syndrome presented as a terminal event and SIADH, usually a secondary phenomenon, was the presenting manifestation. This case also illustrates that SIADH may precede a cancer diagnosis highlighting the importance of exhaustive investigations and close follow-up in patients with resistant hyponatraemia, especially when responsive only to vasopressin receptor antagonists.