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Because it is released in a cohort-based format, the NHID enables longitudinal follow-up and allows investigation of rare endocrine conditions with low population prevalence. Mortality data, including both dates and causes of death, are additionally obtained through linkage with Statistics Korea. Over recent years, use of the NHID has expanded rapidly, establishing it as a major resource for epidemiological research in endocrinology. Nevertheless, because the database was originally developed for administrative and screening purposes rather than for research, investigators face several methodological limitations, particularly the need to construct and validate robust operational definitions of diseases. In this review, we describe the key features of the Korean NHID, summarize operational definitions of endocrine disorders that have been applied in prior research, and provide an overview of recent endocrine studies conducted using this database.<\/jats:p>","DOI":"10.3803\/enm.2026.2953","type":"journal-article","created":{"date-parts":[[2026,2,26]],"date-time":"2026-02-26T06:35:36Z","timestamp":1772087736000},"page":"86-104","update-policy":"https:\/\/doi.org\/10.3803\/crossmark_policy","source":"Crossref","is-referenced-by-count":2,"title":["Nationwide Big Data Studies of Endocrine Diseases Using the Korean National Health Information Database: Research Trends and Standardization of Operational Definitions"],"prefix":"10.3803","volume":"41","author":[{"ORCID":"https:\/\/orcid.org\/0000-0002-7394-3830","authenticated-orcid":true,"given":"Sun 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