A 34 Year Old Woman Presents With Polyuria And Polydipsia

A 34 year old woman presents to your clinic complaining of polyuria and poldipsia of 4 months duration.  She says she has been urinating a 20-30 times per day. Her past medical history is unremarkable. She takes oral contraceptives and occasionally a multivitamin. Her family history is positive for diabetes and heart disease.
Her initial investigations are :

Na = 143 mEq/l
Normal K, cretinine and glucose.
Urine osmolality = 160mOsm/Kg

Q 1 . What is the most likely diagnosis?

Answer: Diabetes insipidus.

( This patient has polyuria with diluted urine and a serum sodium level in the high normal range. A diagnosis of diabetes insipidus can be made if the urine osmolality is less than 250 mOsm/Kg despite hypernatremia. )


Q 2. What are the differential diagnosis? 

Answer: Differential diagnosis include:

• Diabetes mellitus (blood sugar will be high)
• Psychogenic polydipsia
• Salt poisioning ( urine osmolailty will be high) 
• Hypercalcemia

Q 3 . What are the types of Diabetes insipidus and how will you differentiate between them?

Answer: Diabetes insipidus has two types:

• Nephrogenic 
• Neurogenic

When diabetes insipidus is suspected in a patient with polyuria a water deprivation test is done and urine osmolality is monitored. Once the serum sodium reaches 143mEq/l exogenous vasopressin is given. Urine osmolality increases more than 150 mOsm/Kg in patients with neurogenic but not nephrogenic diabetes insipidus. 

Patients with primary polydipsia secrete vasopressin normally so they do not become hypernatremic during diagnostic water deprivation test.