Iodine-131

Iodine-131​ (¹³¹I or I-131) is a radioisotope of iodine, which is used in the diagnosis, treatment and treatmenttheranostic 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.