As thyroxine decreases, so does the speed of metabolism. We then require fewer total Calories. If your thyroid is sluggish, so is your appetite. Conversely, if the thyroid gland is overactive, so is your appetite. The first is stress.
Chronic stress increases thyroid hormone production.
The second is dieting. Calorie deprivation decreases production of thyroid hormone. This causes metabolism to slow down to compensate for the lack of fuel. Individuals with anorexia nervosa, for example, have a basal metabolic rate which is 50 percent below normal. Different tissues and organs of the body also have different metabolic rates.