Long Work Hours Linked to Greater Risk for Hypothyroidism

by Rose Reeb

Increased weekly working hours are associated with a higher risk for hypothyroidism in workers with no evidence of thyroid autoimmunity, according to study results published in Thyroid.

To determine if long work hours are associated with thyroid function, researchers conducted a cross-sectional study using data from 2160 South Korean adults (69.9% men). Individuals who had provided blood and urine samples, were working ≥36 hours per week and <12 hours per day, were not pregnant, did not have a history of thyroid disease, did not have a positive thyroid peroxidase antibody test, did not have isolated hypothyroidism or hyperthyroidism, and were not missing relevant data were included.

Hypothyroidism was defined as a serum thyrotropin level above the upper reference limit with a normal or low free thyroxine level. Hyperthyroidism was defined as a serum thyrotropin level below the lower reference limit with a normal or high free thyroxine level.

The median age of the included individuals was 42.4 years (interquartile range, 33.0- 52.1). The median weekly number of work hours was 47.1 (interquartile range, 39.8-54.9) and 15.4% of included individuals were shift workers. Overall, hypothyroidism was observed in 2.1% and hyperthyroidism was observed in 2.9% of the cohort.

Rates of both hypothyroidism (3.6%) and hyperthyroidism (3.5%) were highest in the group with the highest work hours (P =.03 for trend). The mean number of work hours per week in euthyroid, hypothyroid, and hyperthyroid individuals were 49.5±0.3, 53.4±1.5, and 51.6±1.6 hours, respectively (P =.012). There was a significant association between hypothyroidism and longer work hours per week, even after adjusting for all biological and lifestyle covariates (adjusted odds ratio [aOR], 1.46; 95% CI, 1.12-1.90) and excluding outliers in number of work hours per week (aOR, 1.99; 95% CI, 1.19-3.31) and individuals with overt thyroid dysfunction (aOR, 1.44; 95% CI, 1.11-1.88). Hyperthyroidism was not significantly associated with the number of hours worked per week.

Moreover, hypothyroidism was approximately 2.6 times more prevalent in individuals who worked 53 to 83 hours per week compared with those who worked 36 to 42 hours per week.

As the study was cross-sectional and observational in nature, causation could not be determined. The researchers suggested that “[f]urther research is needed to clarify the causal relationship and the underlying mechanism” of hypothyroidism in connection to longer working hours.


Lee YK, Lee D-E, Hwangbo Y, Lee YJ, Kim HC, Lee EK. Long work hours are associated with hypothyroidism: a cross-sectional study with population-representative data [published online March 31, 2020]. Thyroid. doi:10.1089/thy.2019.0709

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