What is the expected finding in a patient with Chronic Kidney Disease (CKD) stage 4, bone pain, and elevated Parathyroid Hormone (PTH)?

Expected Finding: Hyperphosphatemia (Option B)

In a 10-year-old child with CKD Stage 4, bone pain, and elevated PTH, the expected finding is hyperphosphatemia, not hypercalcemia or elevated vitamin D. 1

Pathophysiology in CKD Stage 4

Phosphate Retention and Elevation

• Serum phosphorus levels begin to rise when creatinine clearance falls below 20-30 mL/min/1.73 m² (CKD Stage 4), despite the maximum compensatory phosphaturic effect of elevated PTH. 1, 2
• Phosphate retention occurs very early in CKD (Stage 1-2), but hyperphosphatemia only becomes evident when GFR declines to this critical threshold of Stage 4. 1
• The elevated PTH in this patient represents a compensatory response attempting to increase phosphate excretion, but at Stage 4, this mechanism has reached its limit and can no longer maintain normal serum phosphorus. 1, 2

Calcium Status

• Hypocalcemia, not hypercalcemia, is the expected finding in CKD Stage 4 with secondary hyperparathyroidism. 1, 3
• The hypocalcemia develops from three interrelated mechanisms: phosphate retention causing calcium-phosphate precipitation, skeletal resistance to PTH's calcemic action, and impaired vitamin D activation. 1
• The elevated PTH is a direct response to this hypocalcemia, attempting to restore calcium homeostasis. 1, 3

Vitamin D Status

• Vitamin D levels are LOW, not high, in CKD Stage 4 patients with secondary hyperparathyroidism. 1, 4
• Impaired kidney function reduces conversion of 25-hydroxyvitamin D to the active 1,25-dihydroxyvitamin D (calcitriol), contributing to hypocalcemia and stimulating PTH secretion. 1, 4
• Approximately 80% of patients with Stage 4 CKD have secondary hyperparathyroidism due to low serum 1,25-dihydroxyvitamin D levels. 4

Clinical Manifestations

Bone Pain Mechanism

• The bone pain in this patient reflects high-turnover bone disease (osteitis fibrosa) caused by excessive PTH-driven bone resorption. 3
• Elevated PTH accelerates osteoclastic activity, releasing calcium and phosphate from bone into circulation, causing abnormal bone remodeling and marrow fibrosis. 3

Laboratory Pattern in CKD Stage 4

• The typical mineral pattern shows: elevated phosphorus (>4.6 mg/dL), low or low-normal calcium, elevated PTH (>110 pg/mL for Stage 4), and low vitamin D. 1, 3
• This constellation of findings distinguishes secondary hyperparathyroidism from primary hyperparathyroidism, where hypercalcemia would be expected. 3, 2

Common Pitfalls

• Do not confuse secondary hyperparathyroidism (low calcium, high phosphate) with primary hyperparathyroidism (high calcium, low phosphate). 3, 2
• The elevated PTH does NOT cause hypercalcemia in CKD Stage 4 because skeletal resistance to PTH and ongoing phosphate retention prevent calcium elevation. 1
• Hyperphosphatemia becomes clinically evident only at Stage 4, even though phosphate retention begins much earlier in CKD progression. 1