• The three significant causes of polyuria and polydipsia include diabetes mellitus, diabetes insipidus (DI), and primary (psychogenic) polydipsia.
• In practical terms, primary polydipsia is a diagnosis of exclusion and should be cautiously established.
• Diabetes mellitus can be ruled out quickly if the blood glucose level is normal.
• Diabetes insipidus is a condition where the body cannot concentrate urine due to either a deficiency of vasopressin (antidiuretic hormone [ADH]) or an abnormal response to ADH. In DI, the plasma sodium concentration is usually >142 mEq/L due to water loss.
• A low serum sodium level combined with initial low urine osmolality makes psychogenic polydipsia the most likely diagnosis.
• Primary polydipsia, also known as psychogenic polydipsia in older literature, is an excessive (pathologic) water drinking condition. It is a psychological disorder more commonly found in women and children. It cannot be diagnosed if there is an underlying cause for the excessive thirst, such as a medication side effect causing "dry mouth" and making them thirsty.
• During a water deprivation test, most patients with primary polydipsia will show a significant increase in urine osmolality. Moreover, patients with primary polydipsia will have low serum sodium levels and osmolality. Restricting water intake normalizes urine output in patients with primary polydipsia.

Notes

• A plasma sodium concentration > 142, coupled with low urine osmolarity and minimal change in urine osmolarity after the water deprivation test, indicates diabetes insipidus.
• If the above conditions are met, but urine osmolality changes less than 10% after a vasopressin injection, it indicates nephrogenic DI.
• If the change in urine osmolality after vasopressin injection is more than 10%, it indicates partial central DI.
• A low serum sodium level, combined with low urine osmolarity and a significant increase in urine osmolality after the water deprivation test, suggests primary polydipsia.

• Psychogenic polydipsia refers to a condition where patients feel compelled to consume massive amounts of free water with little or no solute. This causes expansion of both the ECF and ICF compartments and a decrease in the osmolarity of both compartments. This is an example of hypoosmotic volume expansion; SIADH is another example of this condition.