Thyroid gland is the largest endocrine gland in the body. The thyroid gland produces hormones that regulate the metabolism rate, affect the growth and many functions of various organ systems. Nuclear medicine technique is very useful diagnostic tool in detecting thyroid disease. Thyroid scintigraphy can reveal functional and anatomic information of thyroid gland. A variety of radiopharmaceuticals and methodologies have been used over the years in nuclear medicine. Iodine-131 has been used to image the thyroid several decades. Radioactive Iodine I-131 is useful for the treatment of hyperthyroidism and thyroid cancer. There were 100 participants included in the study who were treated with I-131. The participants were checked 6 months and 12 months after the beginning of the treatment. The results show normalization of the laboratory parameters and hormonal status at the control examinations after 6 months and 12 months therapy with I-131. The side effects did not appear in any of the patients, which demonstrates safe application.