Dear Ahmed,

Thiazide diuretics reduce the poluria of patients. antidiuretic mechanism of thiazides in diabetes ininsipitus is incompletely understood. It is possible that the natriuretic action of thiazides and resulting extracellular fluid volume depletion play an important role in thiazid induced antidiuresis. In this regard whenever extracellular fluid volume is reduced, compansatory mechanism increase NaCl absorbtion in proksimal tubule. consequently, less free water can be formed, which should diminish polyuria. Goodman and Gilman's Pharmacology book page 711.

The principal site of thiazide action is the distal convoluted tubule, where they inhibit the NaCl cotransporter in the luminal membrane, thus decreasing the Cl− and Na+ reabsorption. There is a great Article about this in detail --> Nephrol. Dial. Transplant. (2000) 15 (12): 1903-1905. doi: 10.1093/ndt/15.12.1903

Here is the link to the article

http://ndt.oxfordjournals.org/content/15/12/1903.full

I hope this helps answer your question.

That's a thought provoking question.

Dr. Loffing, in his excellent review (link attached below), writes this ..

// the most widely accepted hypothesis suggests that the antidiuretic action of thiazides is secondary to increased renal sodium excretion . The renal sodium loss causes extracellular volume contraction leading to lowered GFR and increased proximal tubular sodium and water reabsorption. Hence, less water and solutes are delivered to the distal tubule and collecting duct and are lost as urine.// 

Following this statement, he lists out the alternative theories one by one.

http://jasn.asnjournals.org/content/15/11/2948.full

Thiazides are usually prescribed as Diuretics because they inhibit the Na-Cl Cotransporter in the DCT which prevents their reabsorption and increases Osmolarity of Tubular fluid that decreases Water reabsorption and consequently the GFR.

In the case of NDI, our kidneys don't respond to the ADH/Vasopressin hormone. This leads to polyuria as the concentrating ability of the kidney is lost.

We paradoxically give diuretic (Thiazide) to help prevent Polyuria. Here is how it works:

1) Thiazides will increase the conc. of Na and Cl in the Tubular fluid at DCT

2) Macula Densa cells of the Juxta medullary apparatus notice this increase in Na and Cl

3) They release Adenosine

4) Adenosine causes vasoconstriction of afferent arterioles

5) GFR decreases

The decrease in GFR leads to less availability of water for excretion that prevents Polyuria

@Sakthi Amrita OMC - Helped in conclusive answer