<p>The factors influencing the control of body weight are numerous and include many hormones. Even though levels of thyroid hormone might not always be the major factor affecting weight, in the sense that large changes in weight can occur despite a euthyroid state, there are notable changes in weight, appetite and body composition associated with excesses and deficiencies of thyroid hormone. Exploring the effect of thyroid hormone on weight is facilitated by studying the disease states of hypothyroidism and hyperthyroidism, the development and treatment of which can be associated with substantial changes in body weight. As is illustrated in the ensuing discussion, hypothyroidism is associated with modest increases in body weight and accompanying changes in body composition, with reversal of these alterations with its treatment. By contrast, hyperthyroidism can be accompanied by profound weight loss with reversal of the weight loss with restoration of euthyroidism. Using iatrogenic hyperthyroidism, whether during treatment for hypothyroidism or during off-label use in euthyroid individuals, has not proven to be an effective weight loss strategy.</p>