Thyroid Nodules and Obesity

A widely discussed topic in the pathophysiology of thyroid nodules is the role of obesity, a state that leads to increased systemic inflammatory markers. Leptin plays a vital role in forming thyroid nodules and cancer through several mechanisms. Together with chronic inflammation, there is an augmentation in the secretion of tumor necrosis factor (TNF) and the cytokine interleukin 6 (IL-6), which contributed to cancer development, progression and metastasis. In addition, leptin exerts a modulatory action in the growth, proliferation and invasion of thyroid carcinoma cell lines via activating various signal pathways, such as Janus kinase/signal transducer and activator of transcription, mitogen-activated protein kinase (MAPK) and/or phosphoinositide 3-kinase (PI3K)/protein kinase B (Akt). Through several proposed mechanisms, aberrant endogenous estrogen levels have been suggested to play a vital role in the development of both benign and malignant nodules. Metabolic syndrome triggers the development of thyroid nodules by stimulating thyroid proliferation and angiogenesis due to hyperinsulinemia, hyperglycemia and dyslipidemia. Insulin resistance influences the distribution and structure of the thyroid blood vessels. Insulin growth factor 1 (IGF-1) and insulin affect the regulation of the expression of thyroid genes and the proliferation and differentiation of thyroid cells. TSH can promote the differentiation of pre-adipocytes to mature adipocytes but also, in the presence of insulin, TSH possesses mitogenic properties. This review aims to summarize the underlying mechanisms explaining the role of obesity in the pathophysiology of thyroid nodules and discuss potential clinical implications.