One pound of body fat contains about 3500 calories. You must have a deficit of 3500 calories before you actually lose one pound of body fat. If, for example, you require 2000 calories a day to maintain your body weight and you change your diet and lose five pounds in one day, you can see that you are losing water, not fat. It is best to avoid using diuretics unless medically indicated and prescribed by a physician. The thyroid is a gland in the neck that is the major controller of metabolic rate.
Metabolic rate determines the number of calories a person requires per day.
There is a wide range of thyroid levels which are considered normal. An approach that became popular in the 1950's was to give excessive amounts of thyroid to increase a person's metabolic rate and therefore decrease their weight. The approach seems logical, however giving excessive thyroid hormone causes loss of muscle tissue, increased blood pressure, rapid heartbeat, strokes, and even death. Using excessive thyroid medication to artificially increase metabolism results in some loss of fat and marked loss of muscle mass, and causes injury to the heart. This is an extremely dangerous approach and is officially banned by the U.S. Food and Drug Administration for these reasons.