Diabetes insipidus and the water deprivation test

What it is

Diabetes insipidus is a water diuresis. In central DI, ADH secretion is low, that is a decreased ADH release. In nephrogenic DI, ADH secretion is normal but the tubules cannot respond to ADH, which is ADH resistance.

Causes and risk

Central DI is primary and idiopathic, or secondary to brain lesions such as tumors, hypoxic injury and surgery. Nephrogenic DI comes from an ADH receptor mutation, from medications such as lithium and demeclocycline, and from electrolyte disturbances, that is hypercalcemia and hypokalemia. Lithium and demeclocycline will precipitate NDI.

Primary polydipsia, which sits in the same differential as DI, comes from psychiatric diseases such as schizophrenia and obsessive-compulsive disorder, or from lesions in the hypothalamic thirst center, and its mechanism is excessive water intake.

How it presents

Polyuria, nocturia and polydipsia, with hypernatremia and dehydration: the diuresis cannot be stopped, so the blood concentrates. In the absence of nocturia, diabetes insipidus is very unlikely.