A low serum phosphorus concentration is most commonly seen in which scenario?

• A. Patients receiving carbohydrate hyperalimentation
• B. Chronic renal disease
• C. Hypoparathyroidism
• D. Patients with pituitary tumors

Answer: (A)

Think about how phosphate moves between the inside and outside of cells and how the body handles it during feeding. When a malnourished patient suddenly receives a carbohydrate-rich infusion, a surge of insulin is released. Insulin drives phosphate into cells to support glycolysis and ATP synthesis and to build other cellular components. Because the body’s phosphate stores are already depleted, this intracellular shift can drop the serum phosphate level quickly—producing hypophosphatemia. This pattern is classic in refeeding states, such as carbohydrate hyperalimentation.

In contrast, chronic kidney disease usually raises phosphorus levels because the kidneys can’t excrete it efficiently. Hypoparathyroidism tends to cause higher phosphate as well since PTH normally promotes renal phosphate wasting. Pituitary tumors don’t typically produce a low phosphate pattern.