Atypical diabetes insipidus in small cell lung cancer. Paraneoplastic syndrome or metastatic disease?

A 54-year old man was admitted with extensive disease: small cell lung cancer, and severe central diabetes insipidus. Computer assisted tomography of the brain was negative for metastatic spread to the hypothalamus or pituitary gland. Basic levels of antidiuretic hormone were within normal limits, yet the hormone failed to increase secondary to elevated osmotic load. We hypothesize that in this patient, hypothalamus and pituitary gland were morphologically intact, and that diabetes insipidus was induced by the inhibitory action of ectopic opiates on the release of antidiuretic hormone. Nevertheless, since post-mortem studies were refused, metastatic diabetes insipidus could not be definitely excluded, in which case, the source of plasmatic antidiuretic hormone would be the tumor itself.