Thyroxin is produced by your thyroid gland. Thyroxin, being a hormone is something we must be sensitive to. The amount of sensitivity you have toward this hormone is not to be confused with how productive your actual thyroid gland is. Your thyroid may produce a normal amount of thyroxin and yet you can suffer chronic fatigue. The reason for this seeming dichotomy is because we cannot utilize the hormone therefore we have less sensitivity. The opposite is also true.