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Original Article Long working hours and the risk of hypothyroidism in healthy Korean workers: a cohort study
Yesung Lee1orcid , Woncheol Lee1orcid , Hyoung-Ryoul Kim2orcid
Epidemiol Health 2022;44e2022104-0
DOI: https://doi.org/10.4178/epih.e2022104
Published online: November 8, 2022
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1Department of Occupational and Environmental Medicine, Kangbuk Samsung Hospital, Sungkyunkwan University School of Medicine, Seoul, Korea
2Department of Occupational and Environmental Medicine, College of Medicine, The Catholic University of Korea, Seoul, Korea
Corresponding author:  Hyoung-Ryoul Kim,
Email: cyclor@catholic.ac.kr
Received: 1 August 2022   • Accepted: 8 November 2022

OBJECTIVES
Long working hours have been reported to cause various health problems, but are currently practiced in many countries. Building upon a previous cross-sectional study, the authors aimed to elucidate the causal relationship between long working hours and hypothyroidism through a longitudinal study.
METHODS
Data were collected at baseline from 45,259 participants without thyroid disease and with consistent weekly working hours (36-40, 41-52, 53-60, and >60 hours) during the follow-up period. Hypothyroidism was defined using the reference limits of serum thyroid-stimulating hormone and free thyroxine levels. By estimating hazard ratios (HRs) and 95% confidence intervals (CIs) using Cox proportional hazards regression analysis, the risk of incident hypothyroidism was evaluated with 36-40 hours of work per week as the reference.
RESULTS
During 138,261.7 person-years of follow-up, 2,914 participants developed hypothyroidism (incidence density, 2.11/102 person-years). The multivariable-adjusted HRs of incident hypothyroidism for 41-52 hours, 53-60 hours, and >60 hours of work per week were 1.13 (95% CI, 1.03 to 1.24), 2.53 (95% CI, 2.17 to 2.95), and 2.57 (95% CI, 2.09 to 3.15), respectively. In dose-response analyses, long working hours had an approximately linear relationship with hypothyroidism incidence. The risk of incident hypothyroidism in those who worked 53-60 hours and >60 hours per week compared with the reference group was significantly higher among the older age group (≥36 years, stratified by median age), men, and daytime workers.
CONCLUSIONS
This large-scale cohort study demonstrated the association between long working hours and an increased risk of incident hypothyroidism with a dose-response relationship.


Epidemiol Health : Epidemiology and Health