Differences in BMRs are associated with changes in energy balance. Energy balance reflects the difference between the number of calories one eats and the number of calories the body uses. If a high BMR is induced by the administration of drugs, such as amphetamines, animals often have a negative energy balance which leads to weight loss. Based on such studies many people have concluded that changes in thyroid hormone levels, which lead to changes in BMR, should also cause changes in energy balance and similar changes in body weight.

However, BMRs are not the whole story relating weight and thyroid. For example, when metabolic rates are reduced in animals (for example by decreasing the body temperature), they often do not show the expected weight gain. Thus, the relationship between metabolic rates, energy balance, and weight changes is very complex. There are many other hormones (besides thyroid hormone), proteins, and other chemicals that are very important for controlling energy expenditure, food intake, and body weight. Because all of these substances interact with the parts of the brain and body that control energy expenditure and energy intake, we cannot predict the effect of altering only one of these factors (such as thyroid hormone) on body weight as a whole. As a consequence, at this time, we are unable to predict the effect of changing thyroid state on any individual’s body weight