EFFECT OF AGE ON SERUM IMMUNOREACTIVE PARATHYROID-HORMONE IN NORMAL AND OSTEOPOROTIC WOMEN

Abstract

Serum iPTH [immunoreactive parathyroid hormone] was measured in a large series of normal and osteoporotic women as a function of age, with radioimmunoassays using 3 antisera: GP-1M, which recognizes primarily a region within the 44-68 amino acid sequence of PTH; CH-12M, which appears to recognize primarily intact hormone; and CH-14M, which recognizes primarily a region within the 1-34 amino acid sequence of PTH. In normal women 20 to 90 yr of age, serum iPTH increased significantly with age (P < 0.001); the proportional increase was greater when measured with antiserum GP-1M (80%) than when measured with antiserum CH-12M (30%), which suggests that the increase in circulating carboxyl fragments of PTH was greater than the increase in circulating intact PTH. In 40 patients with postmenopausal osteoporosis, the mean value for serum iPTH assayed by antiserum GP-1M did not differ significantly from that for age-matched normal women; however, 3 osteoporotic patients had elevated values and thus appear to represent a separate population. When these subjects were excluded, mean serum iPTH assayed by antiserum GP-1M also was lower than normal (P < 0.001). The mean value for serum iPTH was lower in osteoporotic patients than in normal subjects when assayed by either antiserum CH-12M (P < 0.001) or antiserum CH-14M (P < 0.001). Values for serum phosphate and renal tubular phosphate resorption, both indices of PTH function, were increased (P < 0.005) in the osteoporotic subjects. Although creatinine clearance decreased with age in both normal and osteoporotic subjects, partial correlations with age held constant showed no relationship between creatinine clearance and serum iPTH. A decrease in renal function was probably not the major factor accounting for the rise in serum iPTH with age. Serum iPTH apparently increases with aging but for any given age, it is either normal or low in patients with postmenopausal osteoporosis.