Renal cell carcinoma commonly presents with a paraneoplastic syndrome due to ectopic erythropoietin production. Which lab finding would you expect?

• A. Hyponatremia
• B. Hypercalcemia
• C. Hypocalcemia
• D. Polycythemia

Answer: (D)

Renal cell carcinoma can secrete erythropoietin outside the normal regulatory pathways, leading to secondary polycythemia. When erythropoietin levels rise, the bone marrow ramps up production of red blood cells, so the typical lab finding is an increased red cell mass—elevated hemoglobin and hematocrit. EPO itself would be expected to be high as well, reflecting the ectopic production by the tumor. This constellation often comes with symptoms related to increased blood viscosity, like headaches or plethora.

Other abnormalities like hyponatremia or hyper-/hypocalcemia are not driven by this EPO mechanism. Hypercalcemia can occur with RCC through mechanisms such as PTH-related peptide secretion, but that’s a different paraneoplastic path. The key point here is that ectopic EPO production causes polycythemia due to increased red blood cell production.