ORIGINAL ARTICLE Seasonal variation of serum vitamin D levels in Romania Dan Alexandru Niculescu 1 & Cristina Ana Maria Capatina 1 & Roxana Dusceac 1 & Andra Caragheorgheopol 2 & Adina Ghemigian 1 & Catalina Poiana 1 Received: 4 August 2017 /Accepted: 28 November 2017 # International Osteoporosis Foundation and National Osteoporosis Foundation 2017 Abstract Summary We measured serum vitamin D in 8024 Romanian subjects and found a marked seasonal variation with highest levels in September and lowest levels in March. The seasonal variation (early autumn vs. early spring) persisted in all age and sex groups. The prevalence of vitamin D deficiency was very high. Purpose Romania is located in Eastern Europe, roughly between 44°N and 48°N latitude. Seasonal variation of serum vitamin D in Romanian subjects is unknown. We assessed the seasonal variation of 25-hydroxy vitamin D [25(OH)D] in Romanian population. Methods We retrieved from our endocrinology center database all 25(OH)D measurements between 2012 and 2016. We also evaluated age, sex, diagnosis, and date of blood sampling. The 25(OH)D was measured by two different chemiluminescence or electrochemiluminescence assays. Results There were 8024 subjects (median age 50 (37, 62); 1429 men (17.8%)) without a diagnosis of low bone mass (osteopenia or osteoporosis). The median serum 25(OH)D was 18.6 (12.7, 25.4) ng/mL. Of the subjects, 0.73, 14.4, 55.6, and 86.1% had a serum 25(OH)D level below 4, 10, 20, and 30 ng/mL, respectively. Serum 25(OH)D showed a marked seasonal variation with highest levels in September (24.1 [18.3, 30.3] ng/mL) and lowest levels in March (13.5 [9.4, 19.6] ng/mL) (p < 0.001). The seasonal variation (early autumn vs. early spring) persisted in all age and sex groups and was maximal for 21–40 years of age (26.5 (20.8, 33.1) vs. 12.9 (9.7, 17.9) ng/mL) and minimal for >65 years of age (18.6 (13.0, 27.2) vs. 12.7 (7.8, 19.7) ng/mL). Men and women showed similar amplitude of serum 25(OH)D variation. Conclusion The prevalence of vitamin D deficiency is high, particularly in the elderly. The data show a strong seasonal variation of serum 25(OH)D in all subgroups of our Romanian population with highest levels in September and lowest levels in March. Keywords Vitamin D . Deficiency . Romania . Seasonal Introduction Vitamin D (VD) is increasingly being recognized as an impor- tant factor in bone and general health. However, despite food supplementation in some countries and widely available cholecalciferol-containing medication, vitamin D deficiency and insufficiency are present at a global scale. Serum 25-hydroxy vitamin D [25(OH)D] measurement is widely used for vitamin D status assessment, and low serum 25(OH)D levels are associated with a number of skeletal and non-skeletal disorders [1]. The serum 25(OH)D levels are influenced by many factors such as VD intake, sun exposure (latitude, climate, personal or cultural habits of sun bathing, clothing), and body mass index [2–4]. Despite numerous differences in all these factors among different European countries, there is a very high likelihood of inadequate VD status in all ages but especially in the elderly [5, 6]. Similar results have been reported in Romanian sub- jects (southeastern European country with four seasons, with- out mandatory food fortification with vitamin D) [7–9]. Electronic supplementary material The online version of this article (https://doi.org/10.1007/s11657-017-0407-3) contains supplementary material, which is available to authorized users. * Dan Alexandru Niculescu dan.niculescu@umfcd.ro 1 Department of Endocrinology, Carol Davila University of Medicine and Pharmacy, 34-38 Aviatorilor Blvd., 011863 Bucharest, Romania 2 Research Laboratory, C. I. Parhon Institute of Endocrinology, 34–38 Aviatorilor Blvd., 011863 Bucharest, Romania Archives of Osteoporosis (2017) 12:113 https://doi.org/10.1007/s11657-017-0407-3