Radioactive Iodine I-131 (also called Radioiodine I-131) therapy is a treatment for an overactive thyroid,  a condition called hyperthyroidism.The thyroid is a gland in the neck that produces two hormones  that regulate all aspects of the body's metabolism,  the process of converting food into energy. When a thyroid gland is overactive, it produces too much of these hormones, accelerating the body's metabolism. Radioiodine (I -131) Therapy helps prevent it.

Muscles use something called ATP (adenosine triphosphate) as an energy source. ATP is created in essentially two ways. One is by aerobic metabolism (using oxygen) carried in the bloodstream. This is a sure and steady way to create virtually unlimited sources of energy; however, it has a limit on how fast it can work. The cardiovascular system is limited in its ability to quickly deliver blood and oxygen to the working muscles. Because of this, during high intensity exercise, ATP is also provided by anaerobic metabolism that don't require an ongoing supply of oxygen. Anaerobic pathways can provide ATP immediately, but they also have limited stores that need to be refilled after they are used up. This may cause muscle fatigue.