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

Background: Thyroid disease and obesity are widespread clinical conditions in the population. They can occur together in the same subject, but their relationship does not seem exclusively stochastic. Aim: Literature was critically reviewed to explain the association between thyroid disease and obesity and 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 leading cause of energy expenditure in 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, and TRβ triggers the expression of uncoupling protein 1(UCP1). Despite such mechanisms, there is no current evidence related to the treatment of subjects suffering from obesity with thyroid hormones. Conclusion: Replacement therapy should be reserved only for patients with clear 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)