SEASONAL-VARIATION IN SERUM 25-HYDROXY VITAMIN-D-3 DOES NOT AFFECT 1,25-DIHYDROXY VITAMIN-D

The seasonal variation of serum 25-hydroxy vitamin D-3 and 1,25-dihydr oxy-vitamin D has been investigated. Blood was taken from 27 healthy v olunteers, aged 21-44 years old at 3 monthly intervals over a period o f 1 year. A scrolling monthly programme with 12 quarterly (3 month) ti me periods was developed. A summer associated increase in 25-hydroxy v itamin D-3 was significantly correlated with but lagged behind by 2 mo nths, the increase in recorded sunlight hours. However, four individua ls showed no seasonal rise but maintained constant concentrations thro ughout the year within the established reference range. Serum 1,25-dih ydroxy vitamin D showed marked intra-individual variability with no se asonal pattern although the highest concentration (180 pmol/L) was obs erved in the winter and no concentration greater than 108 pmol/L in th e summer.