Re: “Long Work Hours Are Associated with Hypothyroidism: A Cross-Sectional Study with Population-Representative Data” by Lee et al.

Dear Editor:

Lee et al. performed a study of thyroid function in 2160 subjects in Korea who worked long hours (1). The investigators found a significant incidence of subclinical hypothyroidism that correlated with longer work hours. About 15% of the subjects were shift workers who worked in the afternoon until midnight or worked between 9 pm and 8 am the next day. The diagnosis was based on elevation of thyrotropin (TSH) above the normal range. The time of blood sampling for the thyroid tests was not stated.

Alan Farwell wrote an insightful commentary on these provocative results (2). I propose an explanation for the elevation of TSH that should be considered in relation to working long hours or shift work that relates to sleep deprivation. In a small number of research subjects who had blood sampled for TSH measurement every 20 minutes, Parker et al. showed that sleep deprivation was associated with an increase in the nocturnal peak of TSH that could persist until the next day (3). The mean peak TSH that usually occurs at the onset of sleep increased significantly with sleep deprivation and an elevated TSH persisted into the early morning hours. With sleep reversal defined as staying up all night and sleeping the next day, the peak TSH and the area under the curve was even more dramatically increased. I suggest that sleep deprivation is a possible explanation for the increase of TSH found by Lee et al. in the subjects who worked long hours, especially shift workers.

In further investigations of people who work long hours, as proposed by Farwell (2), the data must include detailed information about the subjects' hours of sleep as well as the specific time of sampling because disturbances of nyctohemeral rhythmicity could explain the elevated serum TSH.