Picture quiz: Diabetes insipidus in a woman with breast cancer

A 43 year old woman was admitted with a two week history of intermittent headache and a three month history of progressive malaise and headache. She had a history of node positive lobular breast cancer, treated with right mastectomy, chemotherapy, and tamoxifen eight years before.

Magnetic resonance imaging of the brain showed a large suprasellar mass (figure). Visual acuity was normal and visual fields were full.

Baseline and dynamic endocrine investigation confirmed anterior pituitary dysfunction affecting the glucocorticoid (morning cortisol low at 21 nmol/l, with adequate adrenal response to synthacten); growth hormone (no response to growth hormone after giving glucagon); and thyroid axis (inappropriately normal thyroid stimulating hormone at 2.76 mU/l with reduced thyroxine at 8.00 pmol/l). Prolactin concentrations were within the normal range at 132 mU/l. She was treated with replacement hydrocortisone and thyroxine.

A few weeks later she was admitted with polyuria and polydipsia. A paired serum concentration of 308 mmol/kg with …