Vitamin D Status and Its Seasonal Variations and Association With Parathyroid Hormone Concentration in Healthy Women in Riga

The aim of the study was to describe the vitamin D status and its seasonal variations in women living in Riga, Latvia, to examine an association between the concentrations of plasma 25-hydroxyvitamin D [25(OH)D] and parathyroid hormone (PTH), and to determine the threshold for plasma 25(OH)D above which there is no further suppression of PTH.
Material and Methods. The data of 189 healthy Caucasian women were analyzed. The serum levels of 25(OH)D, PTH, and phosphorus were measured twice a year. All the participants were divided into 3 groups according to vitamin D supplementation and the reproductive status.
Results. The overall mean level of 25(OH)D was 32.8 ng/mL with significantly lower levels being in winter when compared with those in summer (28.2 ng/mL vs. 37.5 ng/mL, respectively; P<0.05). PTH was negatively associated with 25(OH)D. A threshold level of plasma 25(OH)D above which no further suppression of PTH occurred was found to be 38 ng/mL. Postmenopausal women not taking vitamin D supplements and without exposure to sunlight had 25(OH)D deficiency in winter and summer (92% and 88%, respectively). The most significant seasonal fluctuations were seen in the women of the reproductive age not taking vitamin D supplements and without exposure to sunlight, of which 47% had 25(OH)D deficiency in summer and 69% in winter.
Conclusions. An optimal concentration of 25(OH)D was found to be 38 ng/mL. According to this definition, 70.4% of all the healthy women were classified as vitamin D deficient in winter and 59.8% in summer. The highest proportion of vitamin D deficient individuals was found in the group representing the postmenopausal women not taking vitamin D supplements.