Brief Answer:
See the description

Detailed Answer:
hello,
There is no straight forward answer to this questions, because whatever investigations are available, they are not leading to any particular diagnosis. One important question, was he receiving just olmesartan or olmesartan hydrochlorothiazide combination, you should check on the strips. If he is receiving combination, then answer is clear, the culprit is hydrochlorothiazide.

There are two possibilities, one is vomiting induced and other is idiopathic. They have ruled out the common detectable causes. In elder, also the common cause is idiopathic, meaning that no detectable cause is present. However, some such patients, not all, later detected to have some kind of tumor especially lung and brain tumor. And in rest patients, no cause is found. In these case, there occurs defects in sodium preserving mechanism at micro level.

Also, in our case, another cause could be vomiting induced, for which no treatment is indicated as it has already resolved. So our priority at this stage is to rule out the possibility of those tumors, so he should undergo CECT CHEST AND HEAD. Also, he should undergo ESR, thyroid test, serum cortisol levels in the morning. I think, these things should complete his evaluation of hyponatremia. Otherwise, frequent monitoring of sodium levels as advised and higher salt intake, blood pressure should be monitored as well. Avoid laxative and water pills.

Hope this helps you and get back if you have further doubts.

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