ADH, produced in the supraoptic and paraventricular locations of the hypothalamus, acts on the collecting tubules of the kidney to cause increase in permeability to water and urea. ADH release is triggered by a number of both osmotic and nonosmotic stimuli. Measurement of ADH is useful in separating central diabetes insipidus, which is marked by polydipsia and polyuria and is caused by inadequate ADH production from nephrogenic diabetes insipidus caused by the inability of renal tubules to respond to ADH. In SIADH, release of ADH is disproportionate to a low serum osmolality. SIADH results due to a number of conditions such as pulmonary disease, head trauma, and cancer.

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No isotopes administered 24 hours prior to venipuncture.