ADH release is stimulated by dehydration. ADH can be measured directly in the serum or indirectly by measuring urine osmolality. Normally the urine becomes hyperosmolal with dehydration. If the urine is hypo-osmolal after fluid restriction, hypothalamic diabetes insipidus or nephrogenic diabetes insipidus could be present. In diabetes insipidus, clinical manifestations include thirst, dehydration and polyuria. Administration of exogenous ADH can differentiate between the two, with hypothalamic responding and the nephrogenic unresponsive to the exogenous ADH.