OBESITY, NUTRITION, AND THE THYROID

The concept that ''slow metabolism'' might contribute to obesity, coup led with the idea that thyroid dysfunction is a common cause of ''slow metabolism,'' has led to exploration of the relationship between obes ity, nutrition, and thyroid function, the topic of this review. Some o bese animals have low serum T-3 concentrations; conversely, some studi es of obese humans have revealed high serum T-3 concentrations. Exagge rated 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 a nd 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 adipose mass; long-ter m 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 T-3 may accelerate body weight loss at the expense of excess ive loss of lean body mass and other possible side effects. In conclus ion, abnormalities of thyroid function that are noted in obese individ uals are of uncertain clinical significance. Hypothyroidism is not a s ignificant etiology of idiopathic obesity, and treatment of obesity wi th exogenous thyroid hormones is fraught with significant problems.