Serum TSH measurement is the most sensitive method for identifying thyroid dysfunction  However, few studies have evaluated whether night shift workers develop thyroid diseases, and only a few studies have examined the relationship between TSH levels and night shift work. In a study suggesting TSH levels with sleep time  and another study shows that insomnia is associated with an increased risk of thyroid cancer among postmenopausal women . Moreover, a cross-sectional study of male night shift workers revealed that night shift workers had significantly higher TSH levels, compared to their day shift counterparts .
In the present study, we retrospectively evaluated longitudinal data regarding TSH levels, and found that night shift workers exhibited higher TSH levels, compared to day shift workers, in the age-adjusted annual GLM analyses but not statistically significant.. However, the GEE analyses (adjusted for age and department) confirmed that the TSH levels were higher among night shift workers over the 5-year study period. The GEE analysis was adjusted for department because approximately 70 % of the nurses were night shift workers.
Our results suggest that night shift work might be associated with the risk of subclinical hypothyroidism, and that this risk increased with longer employment as a night shift worker. Subclinical hypothyroid is a condition with normal free T4 levels and elevated TSH levels , which is exclusively diagnosed using screening test results. In the general population, subclinical hypothyroidism has a reported prevalence of 4–15 %, which varies according to the specific study . When anti-thyroid peroxidase autoantibodies are present, there is a 25–50 % risk of subclinical hypothyroidism progressing to overt hypothyroidism within 20 years. If autoantibodies are not present, TSH levels of >3.0–4.5 mIU/L are considered a risk factor for progression, and regular observation is recommended. The current guidelines for the initial treatment of subclinical hypothyroidism recommend starting drug treatment at TSH levels of >10 mIU/L (while considering other co-existing conditions), although it remains unclear whether drug treatment is beneficial for patients with TSH levels of 4.5–10 mIU/L [18–21].
In the present study, we found that age-adjusted TSH levels of ≥4.5 mIU/L among night shift workers during 2011–2015 were associated with a 1.4-fold higher risk of subclinical hypothyroidism, compared to non-night shift workers. In that analysis, we adjusted for age because TSH levels are known to increase with age .
There are several potential explanations for why TSH levels were higher among night shift workers, compared to non-night shift workers. First, TSH levels exhibit a normal circadian rhythm, with study-specific peaks at approximately 2–4 AM and troughs at approximately 4–8 PM . However, this circadian rhythm assumes that workers have a normal sleep at night, and night shift work-related changes in sleep schedule, timing, and quality may alter the body’s normal circadian rhythm and lead to an abnormal TSH circadian rhythm. Furthermore, some authors have suggested that sleep deprivation promotes oscillations in the TSH circadian rhythm , which increases the likelihood that TSH levels rise when workers are deprived of sleep after their night shift.
Second, other studies have found that night shift work disturbs women’s circadian rhythm and induces changes in their female hormone levels , reproductive system , and menstrual cycle . Thus, women might be more sensitive to night shift work-related hormonal changes that could alter TSH levels.
Third, nocturnal eating may affect hormone levels (e.g., TSH, insulin, and glucagon) , and it is possible that night shift work might lead to irregular eating habits and nocturnal eating, which might lead to increases in TSH levels.
Fourth, some studies have also found that night shift work can increase the risk of autoimmune disease and altered immune system function , which might lead to increased TSH levels among night shift workers.
Although the present study was not designed to identify the causal factors that lead to the increased TSH levels among night shift workers, we did observe an increase in TSH levels among night shift workers, compared to non-night shift workers. Furthermore, we assume that night shift work might increase the risk of subclinical hypothyroidism.
The present study has several limitations that warrant consideration. First, we only evaluated female workers at a hospital (as most workers were female and/or nurses), and it is possible that our results may not be observed among men. Second, there was noticeable heterogeneity in the amount and type of night shift work, which included traditional night shifts, day and night shifts, and on-call shifts. This heterogeneity may limit the validity of our analyses. Third, our data were obtained from employee medical check-ups, and it is possible that our data regarding special diseases or drug history might not be accurate. Fourth, we defined the workers’ departments and night shift work statuses based on their status in 2015, and it is possible that not all individuals in the night shift worker group were consistently working night shifts throughout the study period. Fifth, blood samples were obtained at different times, and it is possible that the hormone levels did not reflect circadian rhythm-specific changes, as night shift workers were evaluated during their night shift, while day shift workers were evaluated during their day shift.
Despite these limitations, this study used cross-sectional repeated measures data from a large sample during a 5-year period, which may help overcome these limitations.