Iodine-131

Iodine-131​ (¹³¹I or I-131) is a radioisotope of iodine, which is used in the diagnosis, treatment and theranostic approach (diagnostic scan and radionuclide therapy ³ ) 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 selectively taken up by thyroid follicular cells with the help of sodium-iodine symporters and undergoes trapping and organification. Of note, sodium-iodine symporter cell surface expression is in turn stimulated by thyroid-stimulating hormone (TSH)².

• decay
  • 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. 

History and etymology

The use of radioiodine in clinical practice was introduced in the 1940s by Saul Hertz, MD and director of the thyroid unit of Massachusetts General Hospital ⁴.