Bone Abstracts

FGF-23 is a phosphate regulating hormone and its production may be stimulated by circulating levels of 1,25-dihydroxyvitamin D (1,25-(OH)₂D). Teriparatide administration increases levels of 1,25-(OH)₂D, however it is unclear whether this mediates changes in FGF-23 levels. The aims were i) to determine the effect of teriparatide treatment on circulating levels of FGF-23 and 1,25-(OH)₂D and ii) to describe the time course of effect in postmenopausal women with osteoporosis.

Eighteen postmenopausal women (mean age 65.8 years) with osteoporosis, defined as a DXA BMD T-score of ≤−2.5 at the hip or lumbar spine, received teriparatide (Forsteo 20 μg daily) subcutaneously for 2 years with daily elemental calcium (500 mg) and vitamin D supplementation. Fasting serum was collected at baseline then at weeks 1, 2, 4, 12, 26, 52, 78 and 104. The C-terminal FGF-23 was measured using an ELISA (Biomedica Gruppe) and 1,25-(OH)₂D using an automated immunoassay (iSYS-IDS).

At baseline, mean levels of FGF-23 and 1,25-(OH)₂D were 0.481 pmol/l (95% CI 0.381–0.607) and 60.7 pg/ml (95% CI 52.6–70.2) respectively. At week 1, levels increased significantly from baseline by 20.6% (95% CI 3.1–41.1) for FGF-23 and 86.9% (95% CI 55.1–125.2) for 1,25-(OH)₂D, P<0.001. The increase from baseline in both largely persisted over the 104 weeks of teriparatide treatment, though FGF-23 levels were not significantly different at the final time point.

In conclusion, treatment with teriparatide was associated with early increases in both FGF-23 and 1,25-(OH)₂D. The similar timescale of these changes suggests that the increase in FGF-23 may be mediated, at least in part, by the increase in 1,25-(OH)₂D.