In treatment of papillary carcinomas thyroid, radioiodine destroys the neoplastic cells predominantly by
I131 is treated by the body just like the ordinary nonradioactive isotope, so that when swallowed it is concentrated in the thyroid gland.
It emits mainly β radiation (90%) which penetrates only 0.5 mm of the tissue and thus allows therapeutic effects on the thyroid without any damage to the surrounding structures, particularly the parathyroids.
It also emits gamma rays, which are more penetrating.


Uses

I131is the preferred initial t/t for hyperthyroidism caused by Grave's disease.
It is used in combination with surgery in some cases of thyroid carcinoma, especially those in which metastases are sufficiently differentiated to take up iodides selectively.

Also know,

It is contraindicated in children and pregnant or breastfeeding women, and can induce or worsen ophthalmopathy.
X rays
β rays
γ rays
α particles