Cranial diabetes insipidus masked by anterior pituitary dysfunction in a patient with Sheehan’s syndrome

V Bravis, S Robinson, JPD Cox

Imperial College Healthcare NHS Trust, St Mary’s campus.

Introduction: Sheehan’s syndrome occurs as a result of ischaemic pituitary necrosis due to severe postpartum haemorrhage. It is characterized by varying degrees of anterior pituitary dysfunction. Diabetes insipidus is considered to be an uncommon complication of Sheehan’s syndrome. We present the case of a woman with Sheehan’s and both anterior and posterior pituitary dysfunction, in whom the latter was acutely masked by the former.

Case: A 36-year old woman was admitted to the maternity ward eight days after the delivery of her healthy baby vomiting and feeling unwell after a meal out. Physical examination was unremarkable. She was found to be profoundly hyponatraemic (Na 112mmol/L). Initial IV normal saline treatment led to a drop in Na to 108 mmol/L and a diagnosis of SIADH was made based on this and urinary Na 156 mmol/L with osmolality 445 mOsmol/Kg; 2.7% saline was substituted acutely. Re-visiting the history, she had suffered a significant post-partum haemorrhage. Urgent bloods were sent and hydrocortisone therapy commenced. Bloods revealed hypopituitarism: TSH 0.32mU/L, FT4 7.9pmol/L, LH <0.5IU/L, FSH <0.5IU/L, early morning cortisol 18nmol/L. On treatment Na rose by 8mmol/l per 24 hours. After 48 hours Levothyroxine was commenced. Following this, the patient complained of increased thirst and had a significant diuresis with urine output of 3.7 litres in the first day. Her Na level increased to 144mmol/L and intravenous 5% dextrose infusion was required. She was discharged on full hormone replacement therapy. In clinic, she continued to complain of significant thirst and micturition of 4.5litres per day if DDAVP was not taken. A water deprivation test off treatment was discontinued after showing basal values of plasma osmolality 318 mOsm/kg and urine osmolality 440 mOsm/kg, confirming diabetes insipidus. A repeat test 4 months later, after withdrawal of DDAVP, was normal, showing peak urine osmolality 701 mOsm/kg and plasma osmolality 300 mOsm/kg and confirming resolution.

Discussion: This patient with Sheehan’s syndrome had impairment of neurohypophyseal function, the detection of which was delayed due to anterior pituitary dysfunction at the time, and which eventually resolved. The thirst centre may be affected by ischaemic damage and the osmotic threshold for the onset of thirst in patients with Sheehan’s syndrome is increased. Both deficiencies of cortisol and thyroid hormone impair the body’s ability to excrete free water. If multiple pituitary hormone deficiency is either untreated or undetected, a diagnosis of diabetes insipidus may be missed. Once the missing hormones are replaced, diabetes insipidus may be uncovered.