A 70-year-old woman with metastatic right breast cancer (spinal metastasis) and autoimmune hypothyroidism was referred to the endocrine clinic for thirst and polyuria. She was on prednisolone 10 mg to enhance weight and well-being and letrozole. The oncologist ruled out diabetes mellitus and hypercalcemia. Clinical examination was unremarkable.
Blood investigations revealed the following: normal sodium level, plasma osmolality of 296 mosm/kg, urine osmolality of 161 with low normal urine sodium, and elevated prolactin at 1051 mU/L. Cortisol was checked; however, it cannot be interpreted given her treatment with prednisolone. TSH was suppressed as she was on tbl3 replacement.
The water deprivation tests confirmed the diagnosis of cranial diabetes insipidus. At baseline, her urine osmolality was 161 mosm/kg; after 4 h of water deprivation, it was 288 mosm/kg; and after subcutaneous injection of desmopressin, it further increased to 609 mosm/kg.
She was started on Desmospray which immediately resolved her symptoms.
MRI of the pituitary gland showed a 9-mm enhancing round lesion originating from the pituitary stalk impinging on the optic chiasm and optic tracts suggestive of metastasis (Figure 5).
She received radiotherapy to tbl9 and the whole brain but developed sepsis during her hospital stay. She died 8 months later.