In a word, Uff-da...NO!
When taking HCTZ my sodium readings are always consistently low, as low as 127 early in April. I then drank wine like a crazed wino for 3 weeks and got a 135 which is about as high as it goes.
I KNOW I am extrememly salt sensitive and I also drink a huge amount of liquids, mostly water, diet lemonade, and coffee so I jusge my endlessly low sodium as something of a "water intoxocation" problem. I should test when I am thirsty sometime and see what I get.
My docotr says lower the HCTZ but then he add to try some horrible drug or another...usually amlodipine, until I AGAIN remind him that I cough from ACEI's, ARB's have almost ZERO effect on me, beta-blockers are good drugs only for those with angina or heart failure and Norvasc should only be given to animals...to euthanize them.
Next blood test, I'll go in and tell him I STOPPED EVERYTHING. If we go off on an endless tangent I will add "and that everything includes YOU."
So, I think that one's serum sodium most reflects the amount of water consumed during the day before and maybe the overall condition of the kidneys. I'm beginning to think I should have a closer look at mine; I shouldn't be so salt sensitive and water retentive. Either that or I have frown SERIOUSLY addicted to thiazide.
I don't think one can judge one's BP salt sensitivity from serum sodium levels. Probably the best way would be some sort of standardized test measuring BP response to a certain size "sodium insult"...say 2 hours after 4 grams of ingested sodium (and all the water you want) or some such...or a pound bag of Amish pretzels washed down with diet Pepsi!