There are 3 stages of rickets. Stage I is the early phase where serum calcium is low but serum phosphorous is normal. In stage II, which occurs later, serum calcium concentrations are restored to normal ranges because of compensatory hyperparathyroidism, but serum phosphorous levels are low. In stage III, both serum calcium and phosphorous levels are low and bone disease is florid because of the combined effect of mineral deficiency and hyperparathyroidism. This patient presented in stage II with a normal serum calcium and low serum phosphorous.
Secondary hyperparathyroidism occurs in response to hypocalcemia which occurs in rickets.
PTH acts on the bone, kidney and intestine to have a net effect of increasing serum calcium and decreasing serum phosphorous. PTH acts to increase serum calcium by releasing it directly from the bone. This involves removal of bone and replacing it with osteoid tissue, and in more advanced cases osteoid plus fibrous tissue. Other radiographic findings are resorption of subperiosteal bone, best seen along the margins of the phalanges of the hands. In the skull there may be gross trabeculations or a granular appearance resulting from lytic skull lesions.