Therefore, a lack of ADH prevents water reabsorption and the osmolarity of the blood increases. With increased osmolarity, the osmoreceptors in the hypothalamus detect this change and stimulate thirst. With increased thirst, the person now experiences a polydipsia and polyuria cycle. Hereditary forms of diabetes insipidus account for less than 10% of the cases of diabetes insipidus seen in clinical practice. Diagnosis
To distinguish DI from other causes of excess urination, blood glucose levels, bicarbonate levels, and calcium levels need to be tested. Measurement of blood electrolytes can reveal a high sodium level (hypernatremia as dehydration develops). Urinalysis demonstrates a dilute urine with a low specific gravity. Urine osmolarity and electrolyte levels are typically low.A fluid deprivation test is another way of distinguishing DI from other causes of excessive urination. If there is no change in fluid loss, giving desmopressin can determine if DI is caused by:

a defect in ADH production
a defect in the kidneys response to ADHThis test measures the changes in body weight, urine output, and urine composition when fluids are withheld to induce dehydration. The bodys normal response to dehydration is to conserve water by concentrating the urine. Those with DI continue to urinate large amounts of dilute urine in spite of water deprivation. In primary polydipsia, the urine osmolality should increase and stabilize at above 280 mOsm/kg with fluid restriction, while a stabilization at a lower level indicates diabetes insipidus.
Central DI usually starts between the ages of 10 and 20 and occurs in males and females equally. Nephrogenic DI can begin at any age. The term "diabetes" is derived from the Greek word meaning siphon. Signs and symptoms
Excessive urination and extreme thirst and increased fluid intake (especially for cold water and sometimes ice or ice water) are typical for DI. The symptoms of excessive urination and extreme thirst are similar to what is seen in untreated diabetes mellitus, with the distinction that the urine does not contain glucose. Blurred vision is a rarity. Signs of dehydration may also appear in some individuals, since the body cannot conserve much (if any) of the water it takes in.Extreme urination continues throughout the day and the night. In children, DI can interfere with appetite, eating, weight gain and growth, as well. They may present with fever, vomiting or diarrhea. Adults with untreated DI may remain healthy for decades as long as enough water is consumed to offset the urinary losses. However, there is a continuous risk of dehydration and loss of potassium that may lead to hypokalemia. Cause
The several forms of diabetes insipidus are:

Central
Central (or Neurogenic) DI has many possible causes. According to the literature, the principal causes of central DI and their oft-cited approximate frequencies are as follows:

Idiopathic – 30%
Malignant or benign tumors of the brain or pituitary – 25%
Cranial surgery – 20%
Head trauma – 16%

Nephrogenic
Nephrogenic diabetes insipidus is due to the inability of the kidney to respond normally to vasopressin.