Is there a link between obesity phenotype and thyroid diseases? A mini-review of current concepts

Obesity is a chronic, relapsing, and progressive disease that leads to negative health consequences. Excessive adiposity frequently coexists with metabolic and nonmetabolic complications, deteriorating health and reducing quality of life and life span. Individuals with obesity are not a homogenous group and can present different obesity phenotypes. The most common obesity phenotypes include: metabolically healthy obese (MHO) and metabolically unhealthy obese (MUO). The latter category involves those with developed metabolic syndrome (MetS) and non-fully-developed metabolic syndrome (pre-MetS). The fundamental factor leading to obesity is imbalance between energy uptake and expenditure. From this perspective the thyroid gland plays the pivotal role in metabolism regulation and obesity development. The thyroid regulates thermogenesis, appetite, and lipids turnover. Clinically, hypothyroid patients have decreased metabolic rate and subsequently experience increase of BMI and excess adiposity. The interaction between the thyroid gland and obesity is bidirectional. Several mechanisms of alteration of the hypothalamus-pituitary-thyroid axis in obesity are proposed. Excessive adiposity and dysfunction of adipose tissue may contribute to the development of thyroid functional and structural impairment, such as autoimmunity, thyroid nodules, and thyroid cancer. The prevalence of certain thyroid disorders in obese individuals is higher than in nonobese subjects and this trend is more pronounced in unhealthy obesity phenotypes. The aim of this mini-review is to present the current knowledge on the interaction between thyroid gland disorders and obesity, with special focus on obesity phenotypes.