Long working hours may up hypothyroidism risk

A study from South Korea, presented at ENDO 2020, suggested that individuals who work long hours may be at risk of developing hypothyroidism.

“To our knowledge, this study is the first to show that long working hours are associated with hypothyroidism. Our findings suggest that appropriate monitoring and treatment of hypothyroidism are necessary among individuals who work long hours,” said study author Dr Lee Young Ki from the National Cancer Center, Goyang-si, South Korea.

For this cross-sectional study, researchers used data from the Korea National Health and Nutrition Examination Survey between 2013 and 2015. From this database, they identified 2,160 adults who clocked in 36–83 hours of work each week. Individuals with positive results for thyroid peroxidase antibody (TPO) were excluded.

The prevalence of hypothyroidism was more than twofold among individuals who worked longer hours (3.5 percent vs 1.4 percent for 53–83 vs 36–42 working hours per week). [ENDO 2020, abstract SUN-417]

After adjusting for age, sex, body mass index, urine iodine concentration, smoking status, shift work, and socioeconomic factors, individuals who worked longer hours were at an increased risk of hypothyroidism (odds ratio [OR], 1.46, 95 percent confidence interval, 1.12–1.90 per 10-hour increase in working hours per week).

While the condition is more common in women than men, the researchers noted that the findings in this study were consistent regardless of sex or socioeconomic status.

According to Lee, a majority of the study population had a mild subclinical form of hypothyroidism that was asymptomatic.

Further research is warranted to determine if this association is causal, added Lee. “If long working hours really cause hypothyroidism, the prevalence of hypothyroidism in Korea might decrease slightly as the working hours decrease,” he said, citing a law passed in South Korea in 2018 which reduced the maximum number of working hours per week from 68 to 52.

“If a causal relationship is established, it can [also] be the basis for recommending a reduction in working hours to improve thyroid function among overworked individuals with hypothyroidism. Additionally, screening for hypothyroidism could be easily integrated into workers’ health screening programmes using simple laboratory tests,” he added.

The association between working hours and thyroid disease is not a new one. A previous study comprising 967 women working in a hospital in Incheon, Korea, found that thyroid-stimulating hormone (TSH) levels were elevated in night-shift workers, leading to a 1.399-fold higher risk of hypothyroidism compared with individuals who did not work the night shift. [Ann Occup Environ Med 2016;28:53]

Another study, conducted in 642 individuals in Italy, also showed a higher prevalence of subclinical autoimmune hypothyroidism among shift vs day-time workers (7.7 percent vs 3.8 percent; OR, 2.12). Shift workers were also more likely to have alterations in anti-TPO autoantibodies compared with day-time workers (13.6 percent vs 8.6 percent). [Int J Immunopathol Pharmacol 2006;19(4Suppl):31-36]