Immunoreactive parathyroid hormone (iPTH), calcium, and inorganic phosphorus levels were measured in the serum of 13 apparently normal horses and in six horses with renal insufficiency. Serum iPTH levels were measured by radioimmunoassay using an antisera produced by immunization of guinea pigs with bovine parathyroid hormone. The antisera reacts primarily with the carboxy-terminal hormone fragment. Serum iPTH level in the normal horses was 504.5 ± 191.4 pg/ml and varied from 246 to 750 pg/ml. Serum iPTH levels in three horses with chronic renal failure, hypercalcemia and hypophosphatemia were below the limits of detection for the assay (<165 pg/ml). The hypercalcemia and hypophosphatemia were thus considered not to be the result of increased parathyroid hormone. Three horses with renal insufficiency which were not hypercalcemic had serum iPTH levels which were normal or greater than normal.
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