Bone and Mineral Metabolism, Calcium and Phosphorus
Calcium and phosphate regulation
PTH raises serum calcium, lowers phosphate, and activates vitamin D. Vitamin D increases intestinal calcium and phosphate absorption. Rickets in children causes bowing, widening of wrists, delayed fontanelle closure, and hypocalcemic seizures. Nutritional rickets shows low calcium, low phosphate, high alkaline phosphatase, high PTH, and low 25-hydroxyvitamin D. Hypoparathyroidism causes low calcium and high phosphate; pseudohypoparathyroidism shows resistance to PTH with high PTH. Hypercalcemia suggests hyperparathyroidism, vitamin D toxicity, malignancy, or immobilization.
Traps
Correct calcium for albumin: add 0.8 mg/dL to measured calcium for every 1 g/dL albumin below 4.0. Measure 25-hydroxyvitamin D, not 1,25-dihydroxyvitamin D, to assess stores. Check magnesium in hypocalcemia because hypomagnesemia impairs PTH secretion. In hypocalcemia, check the ECG for QT prolongation and treat symptomatic cases with IV calcium gluconate. In osteogenesis imperfecta, fractures and blue sclerae occur with normal calcium and phosphate; bisphosphonates may reduce fractures.
Worked example
A 1-year-old with bow legs has calcium 8.0, phosphate 3.0, alkaline phosphatase 600, PTH 120, and 25-hydroxyvitamin D 8 ng/mL. This is nutritional rickets; treat with vitamin D and calcium, and check adherence and healing.