Diabetes insipidus (DI) is a syndrome defined by hypotonic polyuria: the passing of large volumes of urine that is more dilute than plasma. It is also defined by a failure to concentrate urine, a task that depends on antidiuretic hormone (ADH, also called arginine vasopressin, AVP) acting on the kidney.
Why does the urine fail to concentrate?
If concentration depends on the hormone and on the kidney, the syndrome can arise in more than one way. Urine concentration needs both enough ADH and a kidney that responds to it, and when either part is missing the urine stays dilute. So DI arises from too little ADH action, in one of two ways:
- the hormone is not secreted: hypothalamic (central) DI;
- the kidney does not respond to ADH: nephrogenic DI.
Fluid intake is a third influence on urine concentration. An intake so large that it suppresses ADH and overrides the kidney’s concentrating work produces dipsogenic DI. The mechanism of the two-part rule is taught in Antidiuretic Hormone Physiology.
Which note explains what?
The satellites run from the hormone to the diagnostic work-up:
- Antidiuretic Hormone Physiology explains where ADH is made and released, how plasma osmolality controls its release, and how ADH concentrates urine in the collecting duct. It is the place to start.
- Clinical Presentation of Diabetes Insipidus describes the syndrome at the bedside: the volume and dilution of the urine and the state of the plasma sodium.
- Hypothalamic (Central) Diabetes Insipidus covers the form in which too little ADH is secreted: its causes, above all pituitary surgery, the triple-phase course that can follow pituitary surgery, and its epidemiology.
- Nephrogenic and Dipsogenic Diabetes Insipidus covers the two forms with normal ADH secretion: renal resistance to ADH, usually from lithium, and the excessive fluid intake of an abnormal thirst.
- Diagnosis and Treatment of Diabetes Insipidus follows the diagnostic pathway from history and laboratory findings to the water deprivation test and plasma AVP measurement, then covers treatment of each form. It is the note to use once the three forms are clear.
Where to start
A reader new to the subject can go in order, starting with the hormone and ending with the diagnostic work-up. A reader who already knows the physiology can start at the form or question of interest. For the first, the opening question is how the body makes and releases ADH, and what tells it when to.