Background: Although 25-hydroxyvitamin D (25[OH]ID) is commonly used to define vitamin D status, there is no consensus on the cutoff levels for vitamin D deficiency and insufficiency. In this study, we aimed to identify the 25(OH)D threshold that maximally suppressed parathyroid hormone (PTH) in Hong Kong Chinese population. Methods: The study included 5276 participants (70% female) of the Hong Kong Osteoporosis Study aged 20 or above who had total 25(OH)D measured. Three-phase segmented regression was used to identify the optimal break-point between 25(OH)D and PTH. Results: The prevalence of vitamin D deficiency observed was 43.8% and the prevalence of insufficient (<75 nmol/L) or deficient (<50 nmol/L) vitamin D levels was 90.1% in our study population. Using unadjusted three-phase segmented regression, the estimated first and second break-point of 25(OH)D on PTH suppression were 32 nmol/L (95% CI: 29-35) and 89 nmol/L (95% CI: 77-101) with an r(2) of 0.048, whereas the estimated first and second break-point of 25(OH)D were 27 nmol/L (95% Cl: 24-30) and 47 nmol/L (95% CI: 37-56) after adjusting for factors affecting bone and mineral metabolism. In addition, the relationship between 25(OH)D and PTH significantly differed by sex and age. Conclusion: The threshold for 250HD at the point of maximal suppression of PTH estimated in this study was lower than the suggested threshold of vitamin D deficiency in the literature, perhaps due to race or assay differences, and the relationship between vitamin D and PTH changed with sex and age. Standardization in the methodology of searching for the optimal break-point is desirable so that a consensus on cutoff points can be obtained. (C) 2017 Elsevier Inc. All rights reserved.