A previously well 49 year old woman was referred to our endocrine service with a compelling 4 month history of polyuria and polydipsia. Her 24 h fluid intake was estimated to be 12 l with 4 l of this taken overnight; these estimates had doubled in the 2 months since referral. Due to the development of an increasingly severe frontal headache, her GP organised a CT head scan which was largely unremarkable aside from a slight fullness to the pituitary gland. Pituitary function tests at this time were normal with elevated gonadotropins. Urgent outpatient investigations including short Synacthen test, MRI pituitary and water deprivation test were arranged. Before any of these investigations were completed, however, the patient was admitted to the acute medical unit 2 weeks after her clinic appointment due to worsening of her symptoms. While monitoring her fluid balance and checking urine osmolality it became clear that she did indeed have diabetes insipidus. Despite being a non-smoker and having no respiratory symptoms, the admission CXR revealed a large right upper lobe mass consistent with a primary lung tumour. An inpatient MRI brain showed multiple metastases including one in the pituitary fossa. Her symptoms responded well to the introduction of both dexamethasone and desmopressin. Prior to discharge she underwent bronchoscopy which confirmed a diagnosis of adenocarcinoma of the right lung. She was ultimately treated with whole brain radiotherapy and palliative chemotherapy. During follow-up she became progressively more Cushingoid due to the high-dose dexamethasone required for symptom control and was diagnosed with steroid-induced diabetes mellitus. This case demonstrates the rarely encountered scenario of central diabetes insipidus due to pituitary metastasis. The possibility of this diagnosis was considered in the first clinic appointment hence the necessity of arranging more urgent investigation than for most suspected/possible DI cases.
16 - 18 Apr 2018
Society for Endocrinology