Both thyroid dysfunction and obesity are highly prevalent in the general population. National data suggest that hypothyroidism is present in 4.6% of the US population, and hyperthyroidism in 1.3%[1]. Obesity rates have climbed in the USA and worldwide over the last several decades; more than 30% of the US population is now classified as obese [2]. This review focuses on recent clinical studies regarding the effects of thyroid treatments on weight change, associations between thyroid status and body weight, and the effects of obesity and weight change on thyroid function.