Immobilisation hypercalcemia.

We studied 5 patients with immobilisation hypercalcemia. Plasma calcium was 3.28 +/- (SD) 0.26 mmol/l. Plasma parathyroid hormone (PTH) level was elevated in 3 patients and inappropriately detectable in 1. Nephrogenous cAMP, measured in 3 patients, was low-normal in 1 who had a normal PTH and zero in 2 who had elevated PTH. Quantitative bone histology was performed in 3 patients. 1 had evidence of increased resorption (osteoclast count 7.1/mm2 section area, normal 1.0 +/- 0.5) and had a high-normal bone formation rate. Another had a normal osteoclast count but no evidence of bone formation. The third had normal bone histology. We conclude that (a) the PTH appears to be biologically inactive and its elevation may be due, in some patients, to renal retention of carboxyterminal fragments; (b) the histologic pattern in bone is variable and not predictable from the duration of illness; (c) the mechanism by which immobilisation induces these changes is obscure, and (d) careful clinical assessment may be necessary to avoid misdiagnosing the condition as primary hyperparathyroidism.