Diabetes insipidus is a condition in which the kidneys cannot conserve water, so the person produces very large volumes of dilute urine (up to about 20 dm3 per day, compared with about 1.5 dm3 normally) and feels extremely thirsty. Unlike diabetes mellitus, the urine contains no glucose; 'insipidus' means tasteless.It is caused by a failure of the ADH system. There are two main types:• Cranial (central) diabetes insipidus: the hypothalamus or posterior pituitary does not make or release enough ADH. It can be caused by a head injury, a tumour, surgery or infection. With little ADH, few aquaporins are inserted into the collecting duct walls.
• Nephrogenic diabetes insipidus: enough ADH is released, but the kidneys do not respond to it. This may be due to faulty ADH receptors or faulty aquaporins, often caused by a gene mutation, or to some drugs such as lithium.In both types the collecting ducts stay nearly impermeable to water, so little water is reabsorbed by osmosis. The blood water potential falls, the osmoreceptors stimulate thirst, and the person must drink large amounts to avoid dehydration.Diagnosis and treatment. In a water deprivation test, fluids are withheld and the urine concentration is measured. In diabetes insipidus the urine stays dilute. A synthetic form of ADH (desmopressin) is then given. If the urine becomes concentrated, the kidneys can respond, so the cause is cranial, and it can be treated with desmopressin. If the urine stays dilute, the kidneys cannot respond, so the condition is nephrogenic and desmopressin will not work.This condition shows how important ADH is in the negative feedback control of water potential.
• Nephrogenic diabetes insipidus: enough ADH is released, but the kidneys do not respond to it. This may be due to faulty ADH receptors or faulty aquaporins, often caused by a gene mutation, or to some drugs such as lithium.In both types the collecting ducts stay nearly impermeable to water, so little water is reabsorbed by osmosis. The blood water potential falls, the osmoreceptors stimulate thirst, and the person must drink large amounts to avoid dehydration.Diagnosis and treatment. In a water deprivation test, fluids are withheld and the urine concentration is measured. In diabetes insipidus the urine stays dilute. A synthetic form of ADH (desmopressin) is then given. If the urine becomes concentrated, the kidneys can respond, so the cause is cranial, and it can be treated with desmopressin. If the urine stays dilute, the kidneys cannot respond, so the condition is nephrogenic and desmopressin will not work.This condition shows how important ADH is in the negative feedback control of water potential.