Brief Answer:
Low sodium

Detailed Answer:
Sorry to learn about your wife's puzzling sodium problem. Low blood sodium can be challenging to diagnose and treat. Sometimes it can be a thyroid or a cortisol problem. When I see someone like her in my practice, I typically order the following blood tests in addition to a detailed physical examination:

CBC (Complete Blood Count, also known as Hemogram; includes Hemoglobin, WBC and Platelet counts)
Electrolytes (Sodium and Potassium in particular)
HbA1c (Glycosylated Hemoglobin = your last 3 months' glucose average). Also known by other names such as GlycoHemoglobin or Glycated Hemoglobin or A1c
Liver function tests (SGOT , SGPT, Albumin, Bilirubin, Alkaline Phosphatase)
Kidney function tests (BUN, Creatinine)
TSH (checks your thyroid)
Free T4 (this checks your thyroid too)                                                                                    Serum Cortisol at 8 am                                                                                     12 hour Fasting Lipid profile
Urine albumin to Creatinine Ratio (early sign of diabetes affecting the kidney)
25 hydroxy Vitamin D   levels   (ideal range 40 to 60 ng/ml =  100 to 150 nmol/liter)     



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Brief Answer:
Follow up

Detailed Answer:
I see.

Well I am quite sure her doctors have ensured she is not on water-pills or a type of medication belonging to the 'ACE inhibitor' category. These are typically used to treat high blood pressure or heart trouble. So if she happens to be on these, then we generally consider stopping these and rechecking the sodium.Obviously, other medications to substitute the ones being stopped is important.
If no cause is evident, it is worthwhile ordering the following tests:

Serum Osmolality by direct osmometer method
Seru Uric Acid
Urine Osmolality (taken at the same time as the blood sample)
Urine Sodium

These tests assist in making diagnosis of a condition called SIADH (Syndrome of Inappropriate Anti Diuretic Hormone Secretion)

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