Ten-year incidence of keratoconus in relation to sex, age, and thyroid gland dysfunction: a nationwide population-based cohort study (2009-2018).

Background: Keratoconus is a corneal ectatic disorder that often leads to visual impairment and may require corneal transplantation. However, its age and gender-based incidence and potential association with thyroid gland dysfunction (TGD) remain poorly understood. This study aims to clarify these aspects and investigate the possible connection between keratoconus and TGD. Methods: We conducted a nationwide population-based cohort study using data from the Korean National Health Insurance Service database. A retrospective chart review was conducted on 4,059,021 patients aged over 20 without underlying corneal diseases in 2009. The end of the review period was at ten years, or until the onset of keratoconus. To evaluate the association with TGD, multivariate Cox regression analysis was used with adjustment of confounding variables such as sex and age. Results: During the review period, 2,334 patients developed keratoconus before the 10-year mark. Females exhibited a higher keratoconus incidence (7.101 per 100,000 person-years) compared to males (5.559) (P<0.001). After adjusting for age, the hazard ratio (HR) for keratoconus was 1.295 times higher [95% confidence interval (CI): 1.193-1.406] in females compared to males. Age groups were stratified in 10-year intervals. The highest incidence of keratoconus was observed in the 20 to 29-year age group (10.695 per 100,000 person-years). All other age groups had significantly lower HR values, with the lowest at 50-59 years (0.508, 95% CI: 0.447-0.577). Keratoconus incidence per 100,000 person-years was 6.227 in subjects without TGD, 6.019 in the hypothyroidism group and 8.287 in the hyperthyroidism group, respectively. Although not statistically significant, individuals with hyperthyroidism showed a higher HR (1.290, 95% CI: 0.939-1.771) for keratoconus when compared to those without TGD, after adjusting for age and sex. Conclusions: This study emphasizes a female predominance in keratoconus incidence and suggests a possible connection between hyperthyroidism and keratoconus. Furthermore, it affirms a higher incidence of keratoconus among young individuals.