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Objective: Although thyroid disease is a common condition, there is limited research examining the prevalence of thyroid disease over a long period of time, including both before and during the COVID-19 pandemic. Therefore, we aimed to investigate sociodemographic aspects that might be associated with thyroid disease and how its prevalence has varied during the pandemic. Methods: We assessed the prevalence of thyroid disease among Korean adults by using data from the Korea National Health and Nutrition Examination Survey from 1998 to 2021. We evaluated weighted prevalence and β-coefficients with 95 % CI for factors such as age, sex, residential area, education level, household income, perceived stress level, weight change, occupation category, and body mass index groups. Results: From 1998 to 2021, the prevalence of thyroid disease among 159,896 Korean adults (88,120 females [55.1 %]) aged 20 years and older exhibited an overall increasing trend. The weighted prevalence in the general population rose from 1.52 % (95 % CI, 1.41–1.64) in 1998–2005 to 3.84 % (3.30–4.39) in 2021, with a higher likelihood of thyroid disease exposure as age increased. In addition, females, individuals with lower education levels, those with high levels of perceived stress, those who gained weight, and those classified as overweight or obese emerged as vulnerable groups for thyroid disease. For the majority of subgroups, the change amid the effect of the pandemic on prevalence was minimal. However, the aged ≥60 years group showed a greater increase in prevalence during the pandemic than before the pandemic (βdiff: 0.52 [95 % CI, 0.37–0.68]). Conclusions: A nationwide representative study in South Korea revealed an increasing trend in the prevalence of thyroid disease over 24 years, particularly among the older population. Despite the minimal variation during the pandemic, our findings emphasize the need for targeted thyroid disease policies and further research, especially for specific subgroups such as the older population.

Background: Although obesity is known to be related to allergic diseases, few studies have investigated the prevalence of allergic diseases in individuals with obesity, especially during the COVID–19 pandemic. Thus, this study aimed to analyze national trends of allergic diseases among individuals with obesity and sociodemographic factors. Methods: This study used data from the Korea National Health and Nutrition Examination Survey to examine the prevalence of allergic diseases among individuals with obesity in South Korea from 2005 to 2021. A nationally representative sample of 118,275 participants aged over 2 years or above was divided into six groups for analysis. This study used weighted multivariate regression analysis to examine the estimates of related factors. It assessed the weighted odds ratios or β–coefficients for these factors across different categories, including age, sex, region of residence, education level, household income, and body mass index for the entire population. Results: All allergic diseases showed a general upward trend from 2005 to 2021, but each disease showed different prevalence trends when compared by age. Before the pandemic, those aged ≤39 years had an increasing trend for asthma and AD, but those aged ≥40 years had a decreasing trend. For asthma, β–coefficients were 0.629 (95 % CI, 0.299 to 0.958) for 19–39 years, −0.245 (−0.450 to −0.040) for 40–59 years, and −0.668 (−1.024 to −0.313) for ≥60 years. For AD, β–coefficients were 2.514 (1.258–3.769) in those aged 2–18 years, 0.630 (0.173–1.086) in those aged 19–39 years, −0.458 (−0.648 to −0.268) in those aged 40–59 years, and −0.253 (−0.454 to −0.052) in those aged ≥60 years. However, for both asthma and AD, there were no significant changes in prevalence during the pandemic. In the case of AR, trends were different from those of asthma and AD. Before the pandemic, AR showed an increasing trend in those aged ≤39 years and those aged ≥40 years: β–coefficients were 3.067 (2.344–3.790) in 19–39 years, 2.051 (1.609–2.493) in 40–59 years, and 1.173 (0.820–1.526) in ≥60 years. During the pandemic, there was an increasing trend only among those aged 40–59, with no significant changes in other age groups: β–coefficients were 1.438 (0.065–2.811) in 40–59 years. Conclusions: From 2005 to 2021, all allergic diseases (asthma, AD, and AR) increased overall, but with different age-related trends. No significant link was found between COVID–19 and allergic diseases, possibly due to preventive measures like mask-wearing and social distancing. Anxiety about accessing healthcare during the pandemic likely contributed to a decline in allergy diagnoses, highlighting the need for comprehensive strategies to manage and prevent allergic diseases.