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.