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The Korean National Health Information Database (NHID) contains big data combining information obtained from the National Health Insurance Service and health examinations. Data are provided in the form of a cohort, and the NHID can be used to conduct longitudinal studies and research on rare diseases. Moreover, data on the cause and date of death are provided by Statistics Korea. Research and publications based on the NHID have increased explosively in the field of endocrine disorders. However, because the data were not collected for research purposes, studies using the NHID have limitations, particularly the need for the operational definition of diseases. In this review, we describe the characteristics of the Korean NHID, operational definitions of endocrine diseases used for research, and an overview of recent studies in endocrinology using the Korean NHID.

Background Long-term glucocorticoid use increases fracture risk by reducing bone mass. This study evaluated the relationship between hip and vertebral fractures and the total amount of systematic glucocorticoid use. Methods We randomly selected 1,896,159 people aged 20 to 100 years who participated in the National Health Checkup program in 2006. The amount of glucocorticoids prescribed was calculated based on the defined daily dose (DDD). The total DDD was obtained by adding oral and parenteral glucocorticoids for 6 months from the index date. Subjects were categorized into four groups according to total glucocorticoid DDDs: non-users (DDDs=0), low users (0< DDDs ≤45), intermediate users (45< DDDs ≤90), and high users (90< DDDs). We followed them for 2 years. A multivariate Cox proportional hazard model was used to evaluate the effects of the total amount of glucocorticoid use on hip and vertebral fractures. Results Higher glucocorticoid use was associated with a higher risk of vertebral fracture. Relative to non-users, the vertebral fracture risk was 1.39 times higher in the low-user group, 1.94 times higher in the intermediate-user group, and 2.43 times higher in the high-user group. The risk of hip fracture was 1.72 times higher in intermediate users and 3.28 times higher in high users than in non-users. Conclusion As the amount of glucocorticoid use for 6 months increased, the risk of hip and vertebral fractures became higher. In order to prevent fractures, it is necessary for doctors to evaluate the total amount of glucocorticoid prescribed to the patient and to provide appropriate treatment.

BackgroundVitamin D deficiency remains common in all age groups and affects skeletal and non-skeletal health. Fibroblast growth factor 23 is a bone-derived hormone that regulates phosphate and 1,25-dihydroxyvitamin D homeostasis as a counter regulatory factor. 1,25-Dihydroxyvitamin D stimulates fibroblast growth factor 23 synthesis in bone, while fibroblast growth factor 23 suppresses 1,25-dihydroxyvitamin D production in the kidney. The aim of this study was to evaluate the effects of vitamin D3 intramuscular injection therapy on serum fibroblast growth factor 23 concentrations, and several other parameters associated with bone metabolism such as sclerostin, dickkopf-1, and parathyroid hormone.MethodsA total of 34 subjects with vitamin D deficiency (defined by serum 25-hydroxyvitamin D levels below 20 ng/mL) were randomly assigned to either the vitamin D injection group (200,000 units) or placebo treatment group. Serum calcium, phosphate, urine calcium/creatinine, serum 25-hydroxyvitamin D, fibroblast growth factor 23, sclerostin, parathyroid hormone, and dickkopf-1 levels were serially measured after treatment.ResultsComparing the vitamin D injection group with the placebo group, no significant changes were observed in serum fibroblast growth factor 23, parathyroid hormone, or dickkopf-1 levels. Serum sclerostin concentrations transiently increased at week 4 in the vitamin D group. However, these elevated levels declined later and there were no statistically significant differences as compared with baseline levels.ConclusionSerum fibroblast factor 23, sclerostin, parathyroid hormone, and dickkopf-1 levels were not affected significantly by single intramuscular injection of vitamin D3.

The majority of the vitamin D in our body is produced by cutaneous synthesis in response to sunlight. As more and more people live in cities and spend the bulk of their time indoors, it can be difficult to get sufficient sun exposure for adequate cutaneous production of vitamin D. Therefore, vitamin D insufficiency has become a very common health problem worldwide. The Korea National Health and Nutrition Examination Survey IV 2008 showed that the prevalence of vitamin D insufficiency, defined as a serum 25-hydroxyvitamin D [25(OH)D] level below 50 nmol/L, was 47.3% in males and 64.5% in females. Only 13.2% of males and 6.7% of females had a serum 25(OH)D level of greater than 75 nmol/L. In Korea, vitamin D insufficiency was more prevalent in young adults than in elderly people, likely due to the indoor lifestyle of younger people. Compared with the United States and Canada, Korea has a lower mean 25(OH)D level and a higher prevalence of vitamin D insufficiency. To improve the vitamin D status of the Korean population, more aggressive policies on food fortification and vitamin D supplementation are needed.

Background Vitamin D plays an important role in the immune response against infection. The purpose of the present study was to investigate the influence of vitamin D deficiency on the progression of otitis media (OM) using an experimental rat model. Methods Four-week-old male Sprague-Dawley rats (n=72) were divided into two groups based on their diet: a control diet group (n=36) and a vitamin D-deficient diet group (n=36). After 8 weeks of diet, experimental OM was induced by inoculation of non-typeable Haemophilus influenzae in the middle ear cavity. The rats were evaluated with otomicroscopy to determine the inflammation in the middle ear mucosa on days 1, 2, 4, 7, and 14 post-inoculation. Bullae from sacrificed rats were collected and analyzed histologically. Results The middle ear mucosa from rats with vitamin D deficiency showed a significantly higher thickness than that of controls during the course of OM. The maximum mucosal thickness was 56.0±9.1 μm in the vitamin D deficiency group, and 43.9±9.8 μm in the control group, although there was no significant difference in the tympanic membrane score between the two groups evaluated with otomicroscopy. An immunohistochemical study showed increased expression of interleukin 6 (IL-6) and tumor necrosis factor α in rats manifesting vitamin D deficiency and decreased expression of IL-10 compared with controls. Conclusion Our results showed that vitamin D deficiency may exacerbate the pathophysiological changes of OM via altered cytokine production. Therefore, maintaining vitamin D status in the optimal range may be beneficial for proper management of OM.

BACKGROUND Vitamin D deficiency remains common in all age groups and affects skeletal and non-skeletal health. Fibroblast growth factor 23 is a bone-derived hormone that regulates phosphate and 1,25-dihydroxyvitamin D homeostasis as a counter regulatory factor. 1,25-Dihydroxyvitamin D stimulates fibroblast growth factor 23 synthesis in bone, while fibroblast growth factor 23 suppresses 1,25-dihydroxyvitamin D production in the kidney. The aim of this study was to evaluate the effects of vitamin D₃ intramuscular injection therapy on serum fibroblast growth factor 23 concentrations, and several other parameters associated with bone metabolism such as sclerostin, dickkopf-1, and parathyroid hormone. METHODS A total of 34 subjects with vitamin D deficiency (defined by serum 25-hydroxyvitamin D levels below 20 ng/mL) were randomly assigned to either the vitamin D injection group (200,000 units) or placebo treatment group. Serum calcium, phosphate, urine calcium/creatinine, serum 25-hydroxyvitamin D, fibroblast growth factor 23, sclerostin, parathyroid hormone, and dickkopf-1 levels were serially measured after treatment. RESULTS Comparing the vitamin D injection group with the placebo group, no significant changes were observed in serum fibroblast growth factor 23, parathyroid hormone, or dickkopf-1 levels. Serum sclerostin concentrations transiently increased at week 4 in the vitamin D group. However, these elevated levels declined later and there were no statistically significant differences as compared with baseline levels. CONCLUSION Serum fibroblast factor 23, sclerostin, parathyroid hormone, and dickkopf-1 levels were not affected significantly by single intramuscular injection of vitamin D₃.

Background The aim of this study was to investigate the prevalence of obesity in Korean men with Klinefelter syndrome (KS) and the associated risk factors for obesity and hyperglycemia. Methods Data were collected retrospectively from medical records from 11 university hospitals in Korea between 1994 and 2014. Subjects aged ≥18 years with newly diagnosed KS were enrolled. The following parameters were recorded at baseline before treatment: chief complaint, height, weight, fasting glucose level, lipid panel, blood pressure, testosterone, luteinizing hormone, follicle-stimulating hormone, karyotyping patterns, and history of hypertension, diabetes, and dyslipidemia. Results Data were analyzed from 376 of 544 initially enrolled patients. The rate of the 47 XXY chromosomal pattern was 94.1%. The prevalence of obesity (body mass index ≥25 kg/m²) in Korean men with KS was 42.6%. The testosterone level was an independent risk factor for obesity and hyperglycemia. Conclusion Obesity is common in Korean men with KS. Hypogonadism in patients with KS was associated with obesity and hyperglycemia.

It has been demonstrated that the vascular risk associated with obesity is particularly correlated with visceral fat. Visceral fat has been shown to secrete more inflammatory cytokines than subcutaneous fat. Inflammatory cytokines, such as interleukin-6, tumor necrosis factor-α, macrophage chemoattractant protein-1, and resistin, induce local and generalized inflammation and may also affect vascular disease [1]. The coronary artery calcium score (CACS) has been shown to predict future cardiovascular events [2]. Previous studies have shown that abdominal obesity is significantly correlated with CACS [3,4]. However, since there are differences in body composition across ethnic groups, it is unclear if such associations are also present in different populations. In this context, I read with great interest the recent article titled "The Relationship of Body Composition and Coronary Artery Calcification in Apparently Healthy Korean Adults" by Yu et al. [5] published in March in the Journal. The authors clearly demonstrated that abdominal obesity as assessed by waist hip ratio is the most significant predictor for CACS in apparently healthy Korean adults. This finding is important, since this is the first demonstration of such an association in a Korean population. More interestingly, they also suggested a contrasting effect of body composition on CACS between genders. Male subjects showed a negative correlation between lean body mass and CACS, while females showed a positive correlation between body fat mass and CACS. However, the authors did not state whether they conducted logistic regression analyses separately for men and women. In my opinion, it would have added more value to their findings if they demonstrated such differences between genders after adjusting for other confounding variables. Another point that should be mentioned is that other metabolic parameters such as blood pressure, total cholesterol, and triglyceride included in the logistic regression analyses were found to not be significant factors for CACS. However, caution should be exercised when interpreting this result. As the authors already mentioned, details on subjects' medical history, including medication use, were not available for the analyses. It is plausible that a significant proportion of the subjects were using medication to control hypertension or hyperlipidemia, which may be why such metabolic parameters were found to be insignificant in the analyses.

Kyeong Hye Park, Kwang Joon Kim1, Han Seok Choi2, Kyoung Min Kim, Eun Young Lee, Seonhui Han3, Hyun Sil Kim3, Daham Kim, Hannah Seok, Eun Yeong Choe, Yumie Rhee, Sung-Kil Lim Department of Internal Medicine, Yonsei University College of Medicine, Seoul; Executive Healthcare Clinic, Yonsei University Health System, Seoul; Division of Endocrinology and Metabolism, Department of Internal Medicine, Dongguk University Ilsan Hospital, Goyang; Department of Oral Pathology, Yonsei University College of Dentistry, Seoul, Korea