The concept that “slow metabolism” might contribute to obesity, coupled with the idea that thyroid dysfunction is a common cause of “slow metabolism,” has led to exploration of the relationship between obesity, nutrition, and thyroid function, the topic of this review. Some obese animals have low serum T3 concentrations; conversely, some studies of obese humans have revealed high serum T3 concentrations. Exaggerated responses of serum TSH following thyrotropin-releasing hormone (TRH) stimulation have also been noted in some human studies, possibly suggestive of thyroid hormone resistance. Thyroid function in humans and animals depends in a complex way on dietary content and composition. Hypothyroid individuals may have transient increases in body weight related to changes in fluid balance rather than adi- pose mass; long-term treatment of hypothyroidism with thyroid replacement in physiologic dosage has little effect on body weight. Hypothyroidism does not seem more common in the obese population than in the general population. In obese individuals, supplementation of weight reduction regimens with exogenous T3 may accelerate body weight loss at the expense of excessive loss of lean body mass and other possible side effects. In conclusion, abnormalities of thyroid function that are noted in obese individuals are of uncertain clinical significance. Hypothyroidism is not a significant etiology of idiopathic obesity, and treatment of obesity with exogenous thyroid hormones is fraught with significant problems.