The traditional TSH and at times a T4 are the accepted tests to determine thyroid function. However, in the case of Obesity, Insulin resistance and Leptin disturbances a patient often have a normal TSH and T4 according to the laboratory ranges, but still have symptoms of hypothyroidism. The body seems to adapt to weight loss by conserving energy. The metabolism does not know the difference between a diet and a famine, even though the person intellectually knows what they are doing, the metabolism is still in the stone age, even though we live in the space age. One of the ways its seems to do this is by causing a hypo-metabolic state by decreasing the conversion of T4 into T3 and the latter also sgets blocked at receptor level. Cells in the pituitary act different than cells in the rest of the body due to Deiodinases. Reverse T3 plays a crucial role here. Some patients find it almost impossible to keep their lost weight off as a result. With the appropriate treatment the rT3 can be decreased and the fT3:rT3 ratio can be increased leading to a more normal metabolic state with resultant increases in success rates in the treatment of obesity. (18953)