Seasonal and age-related differences in serum 25-hydroxyvitamin D, 1,25-dihydroxyvitamin D and parathyroid hormone in patients from Western Norway

Objective. The aim of this study was to investigate the seasonal and age-related variation of vitamin D and PTH serum concentrations in a large general patient population in Western Norway. Design. A retrospective study was conducted at the Hormone laboratory, Haukeland University Hospital, Bergen, Norway. All analyses of 25-hydroxyvitamin D (25(OH) D) (n = 8325), 1,25-dihydroxyvitamin D (1,25(OH)(2)D) (n = 4509) and PTH (n = 4203) requested from private practitioners from 2005 to 2008 were included. All three analytes were available in 1551 subjects. Subjects. Mean age of the study population was 49.8 years and 70.9% of the samples were from women. Results. The highest concentrations of 25(OH) D and 1,25(OH)(2)D were observed in July-September. In April 43% of the studied population had 25(OH) D concentrations below 50 nmol/L. There was a positive correlation between 25(OH)D and 1,25(OH)(2)D (p < 0.001). The levels of 25(OH) D and PTH were negatively correlated (p < 0.001) while 1,25(OH)(2)D and PTH showed a weak positive correlation (p = 0.015). We observed higher concentrations of 25(OH) D (p = 0.003) and lower 1,25(OH)(2)D levels (p < 0.001) in the older age groups. PTH increased throughout the whole age span (p < 0.001). Conclusion. We observed a seasonal variation in 25(OH)D and 1,25(OH)(2)D with low serum concentrations during winter/early spring while PTH showed an inverse pattern. Higher levels of PTH in winter and the elderly may reflect an impaired vitamin D status that may affect calcium homeostasis and bone health.