Thyroid Function and Obesity: From Mechanisms to the Benefits of Levothyroxine in Obese Patients

Abstract

Background: Thyroid disease and obesity are very common clinical conditions in the general population. They can occur together in the same subject, but their relationship does not seem to be exclusively stochastic. Aim: We critically reviewed the evidence of the literature in the attempt to provide explanation for this association, in order to understand the possible benefits of levothyroxine therapy in euthyroid obese patients. Results: A low energy expenditure rate can lead to obesity. Maintaining Basal Metabolic Rate (BMR) is the main cause of energy expenditure for the body, which is regulated by thermogenesis. Thyroid hormone receptors (TR) play different roles in the induction of thermogenetic mechanisms: TRα is fundamental to induce thermogenesis, TRβ triggers the expression of uncoupling protein 1(UCP1). Despite such mechanisms, there is not currently evidence to treat subjects suffering from obesity with thyroid hormones. Conclusion: Replacement therapy should be reserved to patients with obvious signs of subclinical or clinical hypothyroidism. Definitions: Basal metabolic rate (BMR) or basal energy expenditure (BEE): measurement obtained under total inactivity and controlled research conditions; resting energy expenditure (REE): measurement obtained when an individual is sitting quietly (is mildly higher than BMR/BEE).