Vitamin D and parathyroid hormone in relation to bone health in Croatian women.

In N = 400 women (46.9 ± 15.5 years), the prevalence of severe (< 25 nmol/l) and mild (≥ 25 and < 50 nmol/l) 25(OH)D deficiency was 7 and 32.2%, respectively, while 60.8% women had adequate status. Serum iPTH increased (> 37.0 pg/ml) when 25(OH)D was < 46.9 nmol/l. BMD was not significantly lower in the participants with iPTH > 37.0 pg/ml.

PURPOSE: Bone mineral density (BMD) is affected by serum 25-hydroxyvitamin D (25(OH)D) and intact parathyroid hormone (iPTH). Our objective was to analyze the association between serum 25(OH)D and iPTH and their mutual regulatory influence on BMD in women with 25(OH)D deficiency. We also attempted to establish a threshold for the serum 25(OH)D below which serum iPTH increases.

METHODS: The participants were 400 generally healthy women (46.9 ± 15.5 years) recruited from local communities. BMD was measured by dual-energy X-ray absorptiometry, and serum 25(OH)D, iPTH, calcium, phosphorus, alkaline phosphatase, osteocalcin, and C-telopeptide (CTX) were analyzed from the fasting blood samples. Multivariate regression models were used to investigate the relationships among serum 25(OH)D, iPTH, and BMD.

RESULTS: Serum iPTH increased when 25(OH)D concentrations were below 46.9 nmol/l. Severe 25(OH)D deficiency (< 25 nmol/l) was found in 7% of the participants and was associated with significantly higher iPTH compared to participants with serum 25(OH)D ≥ 25 nmol/l. BMD was negatively associated with iPTH only in the group with severe 25(OH)D deficiency. However, BMD was not significantly lower in the participants with severe 25(OH)D deficiency compared to those with mild deficiency or with adequate status.

CONCLUSIONS: Our results showed that threshold level of 25(OH)D below which iPTH started to rise was 46.9 nmol/l. There was no negative impact of iPTH on BMD or bone turnover markers in women with severe 25(OH)D deficiency. This might be due to the relatively young age of our participants in whom the unfavorable effect of 25(OH)D deficiency on bone might have not yet appeared.