I-131 (or 131I) is a radioisotope used in the diagnosis and treatment of thyroid lesions. It is one of the oldest radiotracers used in nuclear medicine, in use for over 50 years. It is predominately used in thyroid ablation therapy, for patients post thyroidectomy, and for metastatic thyroid cancer.
I-131 is taken up by the thyroid cells and undergoes trapping and organification.
- beta emission (which damages the thyroid tissue)
- gamma emission (with a 364 keV photopeak used in imaging)
- half-life: approximately 8 days
The high energy gamma emission leads to poor resolution images. However, the long half-life is beneficial for detecting occult metastatic disease because imaging can be done over a few days after oral administration of the radiopharmaceutical.
I-131 is predominately used in ablation therapy for patients post thyroidectomy for thyroid cancer. Generally, a surgeon performs a near-total thyroidectomy (in order to preserve parathyroid function and due to the inherent difficulty in locating thyroid tissue deep within the neck). Therefore, an I-131 scan is first performed after the patient has had surgery to look for and ablate functioning remnant thyroid tissue.