Purpose: Vitamin D deficiency is a common health problem around the world. This study aimed to evaluate the nationwide prevalence of vitamin D status in tertiary care hospitals in Turkey. Methods: Retrospectively, the data of vitamin D levels from 33 tertiary care hospitals’ clinical biochemistry laboratories around Turkey between 2 January 2016 and 31 December 2016 were conducted. Results: In total 706434 serum samples from adult subjects (F/M: 469028/ 237406; 66.4%/ 33.6%) were included. While vitamin D levels were sufficient in 20.3% (n=14222), they were insufficient in 21.9% (n=154360) and deficient in 57.8% (n=408882). Of the deficient group, 25.0% (176608) had levels <10 ng/mL consistent with profound deficiency. We observed the highest rates of deficiency in those aged between 18-29 years 62.9% (n=70235) and lowest rates in 60-69 years (52.3%, n=61121) and in 70-79 years (52.3%, n=32397). Hypervitaminosis D were consisting of 5.5% of adult subjects. highest rates of hypervitaminosis D were observed in over 80 years (6.6%) and 70-79 years (6.5%), and the lowest in 18-29 years (2.8%). Deficiency rates were 55.4% (n=131468) in men and 59.2% (n=277384) in women. Higher deficiency rates were observed in January (67.9%), February (71.6%), and March (66.7%). Conclusion: In this cohort, over half of the subjects admitted to the tertiary care hospitals in Turkey had vitamin D deficiency, required vitamin D supplementation. The elderly population had the lowest prevalence of vitamin D insufficiency and the highest prevalence of hypervitaminosis D. That may indicate overtreatment of vitamin D supplementation in the elderly group.