Low sodium and chloride in the blood could occur for different reasons.Some medications can lead to this , especially strong diuretics such as Lasix or Demadex or Bumex for example. Other diuretics like HCTZ could contribute to this problem but would be less likely to do this. Diarrhea can be a cause of this problem especially if prolonged. I've seen some people have prolonged diarrhea in response to a class of antidepressants (SSRI's Lexapro, Zoloft, Prozac, Celexa, are common examples)
Certain hormonal changes can cause it. A condition called SIADH which stands for "syndrome of inappropriate antidiuretic hormone" can cause this problem. There are several medical conditions which can lead to this.
Aldosterone is a hormone produced by the body that causes the kidneys to hold on to sodium and chloride and certain meds that block the action of aldosterone can lead to low sodium and chloride. These might include A II blockers, Ace inhibitors, Aldosterone receptor blockers.
Certain types of kidney disease could lead to low sodium and chloride as well.
This covers the major categories of causes . If you have further questions then I'll be happy to get back with you.
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Anthony Bray MD