Is the evidence for ectopic antidiuretic hormone watertight?

The clinical syndrome of inappropriate secretion of antidiuretic hormone (SIADH) associated with malignancy cannot be distinguished from a similar metabolic abnormality found in patients with non-malignant conditions. It is, nevertheless, widely accepted that SIADH associated with malignancy is due to the "ectopic" tumour production of arginine vasopressin (AVP). This view is challenged after scrutiny of available data. Since some malignant or non-malignant tissues may produce neurophysins, the presence of "ectopic" AVP in tumour tissue may be explained by active binding of AVP from the circulation by neurophysins. On the other hand, the absence of detectable AVP in tumour accompanied with SIADH suggests excess AVP secretion from the posterior pituitary or hyponatraemia due to other, as yet unidentified, antidiuretic factors. The concept of "ectopic" AVP may be false.