The Influence of Thyroid Dysfunction on Body Composition and Weight Trajectory

OBJECTIVE: Thyroid dysfunction produces characteristic changes in weight and body composition, but treatment often results in progressive weight gain. METHODS: This review examines underlying mechanisms, predictors, and implications for patient management. RESULTS: There are significant changes in weight, appetite, and body composition associated with underproduction and overproduction of thyroid hormone. The disease states of hypothyroidism and hyperthyroidism can be studied in order to document and understand the significant changes in body weight that ensue with these conditions. In addition, treatment of these conditions is associated with further alterations in body weight. As will be discussed, hypothyroidism is associated with mild to modest increases in body weight and accompanying changes in body composition, with partial reversal of these alterations with its treatment with thyroid hormone. Ongoing treatment of hypothyroidism tends to be associated with ongoing weight gains. In contrast, hyperthyroidism can be accompanied by profound weight loss, with a decrease in fat mass, muscle mass, and bone mass, with reversal of the weight loss with restoration of euthyroidism. Specifically, the transition to euthyroidism with treatment of hyperthyroidism is accompanied by an increase in fat mass, muscle mass, and bone mass. However, resolution of hyperthyroidism typically is associated over time with a net increase in body weight that significantly exceeds the nadir seen during hyperthyroidism. CONCLUSION: Understanding these patterns of weight changes described above is critical for clinicians to appreciate so that prior to treatment patients can be counseled about what to expect, and then after treatment strategies can be developed to prevent or minimize long-term weight gain after restoration of euthyroidism.