In the majority of patients with osteomalacia the serum chemistry is abnormal (Fig. 18.21). When there is malabsorption or dietary lack of vitamin D, serum calcium is decreased or at the lower limit of normal, serum phosphorus is markedly decreased due to secondary hyperparathyroidism, and urine calcium excretion is also decreased. In patients with rickets or osteomalacia due to chronic renal disease, impairment of glomerular filtration leads to phosphate retention so that serum calcium is low because of impaired formation of 1,25- dihydroxyvitamin D and impaired gut absorption of calcium, but serum phosphorus is high. In patients with rickets or osteomalacia due primarily to a renal phosphate leak, serum calcium concentrations may be normal but the serum phosphorus concentration is decreased.
