Hello
A plasma Na concentration less than 135 nmol/L usually reflect a hypotonic state or also known as hyponatremia .
As there are so many causes of hyponatremia but in a 40 years person with hypertension may be these reasons, these include.
1 Heart failure , diagnosis can be confirmed by physical examination as well as E K G ,
ECHOCARDIOGRAM and treadmill test.
Consult a Cardiologist and get his opinion because you are already taking drugs for hypertension.
2 The next reason of such loss of sodium may be due to diuretics ( if you are taking with
high blood pressure drugs)
3
Chronic renal failure as this is most common cause in a patient having high blood pressure.
Diagnosis can be confirmed by M R I of kidneys with
serum creatinine level as blood urea level.
There are so many reasons of low sodium level as hyperlipidemia, hyperproteinemia, hyperglycemia , sweating burns etc .
In my opinion consult a cardiologist and get his opinion.
Hope this information will help you.