Recent compelling epidemiological studies indicate a strong association of obesity with thyroid cancer. Obesity has been shown to promote thyroid cancer progression and exacerbate poor outcome in thyroid cancer patients. However, the molecular mechanisms by which obesity increases thyroid cancer risk and facilitates cancer progression are not completely understood. Obesity induces complex pathological changes including hyperglycemia, hyperinsulinemia, hyperlipidemia, oxidative stress, adipokines, and inflammatory responses. These changes can affect the development and progression of cancer through highly complex interactions in vivo. The deleterious effect of obesity may differ according to the different cancer types. In view of the increased incidence of thyroid cancer in parallel with the widespread occurrence of obesity in the past decades, it is imperative to clarify how obesity affects thyroid carcinogenesis. This review focuses on molecular mechanisms by which obesity aggravates thyroid carcinogenesis as elucidated by mouse models of thyroid cancer.