Abstract
BACKGROUND: Menopause is associated with urine phosphorus retention, which is mitigated by estrogen therapy. Fibroblast growth factor 23 (FGF-23) is a hormone originating from bone that regulates urine phosphorus excretion. Whether sex or estrogen therapy is associated with different FGF-23 levels is unknown. STUDY DESIGN & SETTING: Cross-sectional study of ambulatory individuals with prevalent cardiovascular disease. PREDICTORS: Sex and, in women, use or nonuse of estrogen. OUTCOMES: Serum phosphorus, tubular maximum reabsorption of phosphorus indexed to glomerular filtration rate (TMP/GFR), and plasma FGF-23 concentrations. RESULTS: For 987 participants, mean age was 67 ± 11 years, 182 (18%) were women, and 46 (25%) were using estrogen. Mean estimated GFR was 71 ± 23 (SD) mL/min/1.73 m(2). Compared with women who were not using estrogen, both women on estrogen therapy and men had significantly lower serum phosphorus concentrations, lower TMP/GFR values (indicating higher urine phosphorus excretion), and lower FGF-23 concentrations with adjustment for age, demographics, and kidney function (P < 0.001 for each). Mean FGF-23 levels were 68.7 (95% CI, 59.7-79.0) relative units (RU)/mL in non-estrogen-using women, 43.8 (95% CI, 41.2-46.5) RU/mL in men, and 45.1 (95% CI, 35.2-57.4) RU/mL in women using estrogen in adjusted analysis (P < 0.001). LIMITATIONS: Most participants were men. Estrogen therapy was not randomly assigned. CONCLUSIONS: Older women who are not using estrogen have higher FGF-23 levels than either men or women using estrogen. In the context of prior literature, these data suggest that postmenopausal phosphorus retention may stimulate higher FGF-23 concentrations after menopause.