diabetes insipidus

Diabetes insipidus is a condition of an “leaky tap” for water: the kidneys cannot hold onto water, so the body produces huge volumes of pale, dilute urine and the person becomes extremely thirsty. Despite the shared word “diabetes,” it has nothing to do with blood sugar — the name comes from the large urine flow, which here is tasteless (“insipid”) rather than sweet.

There are two main forms. In central diabetes insipidus the pituitary makes too little vasopressin, often after head injury, surgery, or a tumor affecting the hypothalamus or posterior pituitary. In nephrogenic diabetes insipidus the hormone is present but the kidney cannot respond, due to inherited channel or receptor defects, certain drugs (notably lithium), or low potassium or high calcium. Either way, water is not reabsorbed and pours out.

People typically pass several liters of dilute urine a day and drink constantly to keep up; if they cannot drink, blood sodium rises dangerously. The two forms are told apart with a water-deprivation test and a trial of synthetic vasopressin (desmopressin): the central form responds to the drug, while the nephrogenic form does not. This neatly mirrors whether the problem is too little hormone or an unresponsive kidney.