By Sally Robertson, B.Sc.Mar 31 2020

A new study has found that people who work long hours are more likely to have an underactive thyroid or hypothyroidism than people who work fewer hours.

Image Credits: aslysun / Shutterstock.com

The findings showed that the association between working long hours and an increased prevalence of hypothyroidism occurred irrespective of gender or socioeconomic status, even though the condition is usually more prevalent among women than men.

The study, which the principal investigator says is the first to demonstrate this association, has been accepted for delivery at the Endocrine Society’s annual meeting – ENDO 2020 – and has also recently been published as part of special supplement in the Journal of the Endocrine Society.

Overwork is threatening people’s health

"Overwork is a prevalent problem threatening the health and safety of workers worldwide," says Young Ki Lee, from the National Cancer Center in Goyang-si, South Korea. "To our knowledge, this study is the first to show that long working hours are associated with hypothyroidism."

Hypothyroidism is a condition where the thyroid gland in the neck (in front of the windpipe) fails to produce enough thyroid hormones, which are crucial for metabolism and protein synthesis. The condition is usually caused by the immune system targeting the thyroid gland or by certain treatments for an overactive thyroid (hyperthyroidism) or thyroid cancer.

The two main hormones the thyroid gland produces are thyroxine and triiodothyronine. When levels of thyroid hormones are low, the body’s metabolism slows down.

Signs and symptoms of the condition vary, depending on the degree of hormone deficiency. People may not experience any symptoms in the early stages or simply attribute them to aging, but eventually, hypothyroidism can lead to a range of health conditions. Examples include weight gain and obesity, diabetes, fatigue, joint pain, depression, infertility, muscle weakness, and heart disease.

Approximately one to two percent of people develop hypothyroidism and the condition is ten times more common among women than among men.

What did the study involve?

For the study, Lee and colleagues pooled data for 2,160 adults working full time who participated in the Korea National Health and Nutrition Examination Survey between 2013 and 2015. An underactive thyroid was identified using records of blood samples taken from the participants.

The researchers found that among people who worked between 52 and 83 hours per week, the prevalence of hypothyroidism was more than double that among people who worked a 36- to 42- hour week, at 3.5% versus 1.4%.

According to Lee, for each additional ten hours people worked per week, the risk for hypothyroidism was 46% greater compared with people who worked ten hours less per week.

Lee says more research is needed to establish whether working long hours causes the condition to develop.

A reduction in working hours may be recommended

"If a causal relationship is established, it can be the basis for recommending a reduction in working hours to improve thyroid function among overworked individuals with hypothyroidism,” says Lee. “Additionally, screening for hypothyroidism could be easily integrated into workers' health screening programs using simple laboratory tests."

In 2018, a law was introduced in South Korea that lowered the maximum number of hours a person is allowed to work each week from 68 to 52.

If long working hours really cause hypothyroidism, the prevalence of hypothyroidism in Korea might decrease slightly as the working hours decrease."

Young Ki Lee, National Cancer Center in Goyang-si, South Korea

Although hypothyroidism can eventually lead to symptoms such as weight gain, fatigue and depression, Lee reports that in most of the study cases where hypothyroidism was identified, people had a mild form of the disease that had not yet become symptomatic.