The mediating role of thyroid hormone in the association between heatwave exposure and depression severity

High-temperature exposure has been reported to be associated with depression scores and the risk of hospital visits. However, how heatwaves affect depression severity and the underlying mechanisms remains unclear. Inpatients with depression from the Anhui Mental Health Center in China between January 2020 and December 2023 were included in this retrospective study. The patients were grouped into mild, moderate and severe depression categories according to discharge diagnoses. The effects of heatwaves in two weeks prior to admission on depression severity were assessed based on mixed-effect logistic regression model, and the roles of thyroid hormones were quantified by piecewise structural equation modeling. Stratified analyses were conducted by gender, age, BMI and the COVID-19 pandemic. A total of 5,978 inpatients with depression were hospitalized 7,118 times during the study period. The depression severity escalated with the threshold and duration of heatwaves. The risk of more severe depression during heatwave exposure (≥95^th percentile, ≥3 days) was 12.4 % (95 % CI: 6.1 %, 19 %) higher compared to non-heatwave conditions. The proportions of mediating effects of thyroid hormones (T3, FT3, T4 and the FT4/FT3 ratio) were 7.7 % (95 % CI: 0.3 %, 20.1 %), 13.6 % (95 % CI: 1.5 %, 30.6 %), 12.8 % (95 % CI: 4.3 %, 25.7 %) and 23.3 % (95 % CI: 11.4 %, 54.1 %) in the influences of heatwaves on depression severity, respectively. Males, young and middle-aged patients, overweight patients, and those hospitalized during the COVID-19 pandemic, were all more sensitive to heatwaves. Our study showed that heatwaves significantly correlated with depression severity. Thyroid hormones may played potential mediating roles in this association.