This is the solution to a case we presented recently for readers to solve. First, here is a quick review of the case:
A 70-year-old man with non-small-cell lung cancer presented to the clinic with fatigue and dizziness for 1 month. He complained of weight loss (5-10 pounds) and anorexia, and his wife said he seems forgetful. His blood pressure was 95/47 without orthostasis or tachycardia. He did not have peripheral edema, and the remainder of the exam was unremarkable.
These were his lab values: sodium 115 mEq/L; potassium 6.1 mEq/L; chloride 83 mEq/L; bicarbonate 20 mEq/L; blood urea nitrogen (BUN) 28; creatinine 1.4 mg/dL; glucose 72 mg/dL; Serum osm 243 mOsm/kg; urine osm 343 mOsm/kg.
Solution and Discussion
I have a standard method for evaluating serum sodium. I will present a stepwise system that I encourage you to use. Internal medicine requires some compulsivity, and electrolyte disorders require you to work through the possibilities carefully.
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Now we can begin to draw some conclusions:
- The decreased serum osms confirm true hyponatremia. (Our first step is to exclude pseudohyponatremia.)
- The next step is to assess volume status. The low blood pressure in our patient suggests volume contraction; however, the lack of orthostasis and tachycardia weigh against this.
- The patient has no edema; thus, we either have a volume contraction-associated hyponatremia or a euvolemic hyponatremia. Assuming the latter, one should first consider hypothyroidism, Addison's disease, and chronic kidney disease.
- The hyperkalemia is the second clue. It is accompanied by a decreased bicarbonate level, suggestive of a type IV renal tubular acidosis (RTA).
- Type IV RTA is caused either by hypoaldosteronism or by a drug that interferes with aldosterone.
- While lung cancer often causes SIADH, one should suspect adrenal metastases in this case. The patient has hyponatremia and hyperkalemia plus signs and symptoms of corticosteroid insufficiency.
- For completeness, we obtained a TSH (thyroid-stimulating hormone) level, which was normal. We did an ACTH (adrenocorticotropic hormone) stimulation test, finding a low cortisol level (3.1), which did not increase with ACTH stimulation.
- An abdominal CT scan revealed bilateral adrenal metastases.