In a pediatric case with low calcium and phosphorus, high alkaline phosphatase and high parathyroid hormone, along with low 25-OH vitamin D and low 1,25(OH)2 vitamin D, which diagnosis is most likely?

In a pediatric case with low calcium and phosphorus, high alkaline phosphatase and high parathyroid hormone, along with low 25-OH vitamin D and low 1,25(OH)2 vitamin D, which diagnosis is most likely?

• A. Vitamin D deficiency rickets
• B. Renal tubular acidosis
• C. Osteogenesis imperfecta
• D. Vitamin D dependent rickets type 2

Answer: (A)

This pattern reflects vitamin D deficiency causing rickets. When vitamin D is insufficient, the gut absorbs less calcium and phosphate, leading to low calcium and phosphate in the blood. The drop in calcium triggers secondary hyperparathyroidism, so parathyroid hormone rises and drives bone turnover, which shows up as a high alkaline phosphatase. Parathyroid hormone also promotes renal phosphate wasting, keeping phosphate low. Because 25-hydroxy vitamin D is the circulating reservoir, its deficiency means there’s less substrate to form the active 1,25-dihydroxy vitamin D, so that level is also low. This combination—low calcium and phosphate with high PTH and high alkaline phosphatase, plus low 25-OH vitamin D and low 1,25(OH)2 vitamin D—is most consistent with vitamin D deficiency rickets. Renal tubular acidosis would typically present with metabolic acidosis and a different pattern of mineral disturbances. Osteogenesis imperfecta involves bone fragility with normal mineral metabolism labs. Vitamin D dependent rickets type 2 involves end-organ resistance to 1,25(OH)2 vitamin D, where 1,25(OH)2 vitamin D is often normal or elevated despite hypocalcemia, not low as seen here.

This pattern reflects vitamin D deficiency causing rickets. When vitamin D is insufficient, the gut absorbs less calcium and phosphate, leading to low calcium and phosphate in the blood. The drop in calcium triggers secondary hyperparathyroidism, so parathyroid hormone rises and drives bone turnover, which shows up as a high alkaline phosphatase. Parathyroid hormone also promotes renal phosphate wasting, keeping phosphate low. Because 25-hydroxy vitamin D is the circulating reservoir, its deficiency means there’s less substrate to form the active 1,25-dihydroxy vitamin D, so that level is also low. This combination—low calcium and phosphate with high PTH and high alkaline phosphatase, plus low 25-OH vitamin D and low 1,25(OH)2 vitamin D—is most consistent with vitamin D deficiency rickets.

Renal tubular acidosis would typically present with metabolic acidosis and a different pattern of mineral disturbances. Osteogenesis imperfecta involves bone fragility with normal mineral metabolism labs. Vitamin D dependent rickets type 2 involves end-organ resistance to 1,25(OH)2 vitamin D, where 1,25(OH)2 vitamin D is often normal or elevated despite hypocalcemia, not low as seen here.