Age-Related Changes in Serum Immunoreactive Parathyroid Hormone and its Biological Action in Healthy Men and Women*

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Abstract
Circulating levels of PTH and related parameters of calcium and phosphate metabolism were measured in healthy free-living elderly and young subjects residing in the Southwest of determine if parathyroid function changes with aging. Serum immunoreactive PTH (iPTH) was measured with two well characterized antisera; an amino (N)-terminal anti-serum which cross-reacts with the biologically active domain (1-34) and recognizes intact hormone, and a midregion (44-68) antiserum which cross-reacts with intact hormone and biologically inactive midregion/C-terminal fragments. Serum iPTH in both RIAs was significantly increased in the elderly population. An age-related increase was also found for total urinary cAMP and serum alkaline phosphatase, whereas the tubular reabsorptive maximum for phosphate (TmP/GFR) decreased with age. No difference was found between men and women of the same age group for serum iPTH, urinary cAMP, or serum alkaline phosphatase. TmP/GFR declined with age in men, but not women. Correspondingly, serum phosphate was significantly lower in elderly men than in elderly women. Urinary calcium excretion was higher in elderly women than in men of the same age group. Neither serum total or ionized calcium decreased with age. In conclusion, the age-related increase in N-terminal PTH and alterations in associated parameters of phosphate and calcium metabolism are consistent with increased parathyroid function as men and women age. Factors other than PTH are responsible for the sex-related differences observed in TmP/GFR, calcium excretion, and serum phosphate. The cause of the increased circulating levels of apparently biologically active PTH is unclear, but extends beyond the age-related decrease in renal function.