Regulation of thyroid function by lifestyle factors and its implication on energy homeostasis and cardiometabolic health in young euthyroid adults

Abstract

The aims of this Doctoral Thesis are to analyse, in young euthyroid adults, the relationship between habitual nutrient intake, dietary patterns, physical activity levels and sleep habits and thyroid function in young euthyroid adults (Study 1); the association of thyroid function with body composition and cardiometabolic risk (Study 2) and the effect of a cold exposure on thyroid function and the association between thyroid function and BAT volume and function (Study 3). The results of the present Doctoral Thesis show that lifestyle factors such as dietary energy intake, is directly associated with thyroid function, which is inversely related to adherence to the Mediterranean diet and physical activity levels in young euthyroid adults. These results also show that free T3 was positively associated with central adiposity and cardiometabolic risk factors, such as insulin resistance, fatty liver index and blood pressure, even in young euthyroid adults. Lastly, despite the serum thyroid-stimulating hormone and free T4 levels decrease in response to a personalized mild-cold exposure, thyroid function is not associated with BAT volume or function in young euthyroid adults. Collectively, the results of this Doctoral Thesis increase our knowledge about the lifestyle determinants of thyroid function in young euthyroid adults, and the relationship of thyroid function with cardiometabolic health and BAT metabolism. Dietary habits and physical activity appear to be associated with thyroid function in healthy individuals. On the other hand, even in euthyroid individuals, thyroid function seems to be linked to central adiposity and cardiometabolic risk, although no association seems to exist between thyroid and BAT metabolism in young euthyroid adults.