CASE FILES: BIOCHEMISTRY
A N SW E R S TO C A SE 46: D IA B E T E S IN SIP ID U S
A 32-year-old male who had suffered head trauma is in stable con-
dition in the ICU with increasing dilute urine output.
Diagnosis: Diabetes insipidus.
Biochemical mechanism: Absence of vasopressin leading to inability
to retain free water.
C L IN IC A L C O R R E L A T IO N
Vasopressin is a hormone produced in the hypothalamus and stored in the pos-
terior pituitary gland. It is also called antidiuretic hormone, because its presence
stimulates water resorption in the distal renal tubule. Excess antidiuretic hor-
mone can lead to “water intoxication” and hyponatremia. In contrast, lack of
vasopressin leads to excess loss of free water and hypernatremia. The clinical
presentation is that of a patient who is excessively thirsty, having to drink large
amounts of water, and urinating large amounts of dilute urine. Head trauma is
one of the most common causes, particularly if the posterior pituitary stalk is
disrupted. Excessive water (psychogenic water drinking) may present similarly,
but these individuals will have normal decreased urine response to water
restriction, and during sleep. In contrast, patients with diabetes insipidus (DI)
will have excessive urinary loss even with water restriction and even during the
night. The treatment is administration of desmopressin acetate (DDAVP), a
synthetic analogue of AVP.
A PPR O A C H TO V A SO PR E SSIN A N D W ATER B A L A N C E
Understand the role of vasopressin and control of water metabolism.
Know the role of aldosterone in regulating salt and water balance.
Be aware of hormones that regulate salt and water balance (renin;
angiotensin I, II, III).
Aldosterone: A mineralocorticoid hormone that is synthesized from cho-
lesterol in the adrenal cortex and released in response to angiotensin II
or III. It increases the ability of the kidney to absorb Na+, Cl-, and water
from the glomerular filtrate.
Angiotensin converting enzyme: An enzyme found primarily in the lung
(as well as the vascular epithelium and other tissues) that removes two
amino acids from angiotensin I to form angiotensin II.