I am sorry that your question was unanswered for long.
An infection like pneumonia can be responsible for all his symptoms and complications. The feeling of fullness, bloating or loss of appetite occurs in infection. Pneumonia can fill the fluid in the lungs (called pleural effusion or pulmonary edema). Following condition can cause sodium deficiency;
1) High blood sugar (diabetes)
2) Hyperlipidemia (high cholesterol and triglycerides)
3) Paraproteinemia; increased proteins in the body
4) Medications like diuretic
5) Kidney diseases affecting renin-angiotensin system
6) Antidiuretic hormone (ADH) secretion
7) Salt-wasting nephropathy (kidney disease)
8) Liver cirrhosis,
9) Congestive heart failure,
10) Nephrotic syndrome,
11) Nonsteroidal anti-inflammatory drug (NSAID) use may increase the risk of development of hyponatremia
12) Excessive exercises
Investigations to assess the causes for low sodium are;
1) Urine osmolality
2) Serum osmolality
3) Urinary sodium concentration
4) Serum albumin,
5) Serum triglycerides,
6) Serum protein electrophoresis
7) Thyroid-stimulating hormone (TSH)
8) Serum cortisol levels
9) Serum uric acid levels
The treatment is dependent on the severity of deficiency of sodium, mode of onset and cause. If it is severe and sudden, intravenous saline drips is required, otherwise oral increased table salts with periodic monitoring of the blood levels of the sodium and management of the cause (like kidney disease will need dialysis) takes care of the deficiency. In his case a combination of congestive heart failure, pneumonia and kidney disease are the likely possibility of the low sodium. With the antibiotics, infection would be controlled and gradually rest of the complication also gradually are likely to resolve.
Please feel free for your follow up questions.
I would be happy to assist you further, if you need any more information.
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