Association between thyroid function and acute mountain sickness upon rapid ascent to 3650 m in euthyroid lowlanders in China

Acute mountain sickness (AMS) is a major health issue for lowlanders when they ascend rapidly to altitudes above 2500 m. Thyroid hormones are essential for adaptive responses to the hypoxic environment of high altitude. However, there is limited evidence regarding the association between thyroid function and the prevalence of AMS. This study included 70 healthy euthyroid Chinese lowlanders who ascended from Beijing (44 m above sea level) to Lhasa (3650 m above sea level) by aircraft (flight time, approximately 5 h). The dataset was divided into a training set (80%) and a validation set (20%) for the development and validation of a prediction model. Thyroid hormones, demographic data and blood biochemical data were measured in the week before the ascent. The subjects in the training set were divided into an AMS group and a non-AMS group based on the 2018 Lake Louise Acute Mountain Sickness Scale score. Thyroid hormones and thyroid hormone sensitivity indices were compared between the groups. Multivariable logistic regression analysis revealed that a higher baseline total triiodothyronine (TT3) level (odds ratio (OR) 2.474, 95% confidence interval (CI) 1.083-5.653) and a higher free triiodothyronine/free thyroxine (FT3/FT4) ratio (OR 3.427; 95% CI 1.266-9.280) were independent risk factors for development of AMS. The receiver-operating characteristic and calibration curves showed that the model had good predictive ability and consistency in both the training and validation sets. In China, euthyroid lowlanders with a higher TT3 level or FT3/FT4 ratio are more susceptible to AMS after exposure to high altitudes.