Effect of Age on Serum Immunoreactive Parathyroid Hormone and Its Biological Effects*

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Abstract
Immunoreactive parathyroid hormone (iPTH) levels, nephrogenous cAMP (ncAMP), and tubular maximum phosphate reabsorption (TmP) were measured in 10 young and 12 healthy volunteers. The fasting urinary calcium to creatinine ratio (Ca:Cr) was also quantitated as an index of bone resorption. Aging was attended by increased iPTH levels (6.9 ± 0.8 us. 3.4 ± 0.4 /¼leq/ml; P < 0.01) as well as increased ncAMP levels (2.48 ± 0.28 vs. 1.12 ± 0.21 nmol/100 ml glomerular filtrate; P < 0.005) and decreased TmP (2.9 ± 0.2 vs. 4.1 ± 0.2 mg/100 ml glomerular filtrate; P < 0.005), indicating that the increased iPTH levels reflected the biological effects of the hormone. A significant positive correlation of iPTH and ncAMP and a significant negative correlation of iPTH and TmP were observed. The Ca:Cr was increased in the older volunteers (0.10 ± 0.02 vs. 0.05 ± 0.01; P < 0.05). The elderly subjects had significantly decreased daily calcium ingestion, serum phosphate and albumin, and creatinine clearance. Our findings suggest that the increased biological effects of PTH in the elderly subjects may contribute to the increases in Ca:Cr and bone loss that occur with age.