How are doses of radioactive iodine chosen?

Doses of radioactive iodine used to treat patients with Graves’ disease may be selected based on different criteria. In the setting of Graves’ disease, the overall goal of treatment with radioactive iodine is to administer a dose of 131-Iodine that will deliver enough beta radiation to destroy a sufficient number of the overactive follicular cells that produce and secrete excess amounts of thyroid hormone. If a dose of 131-Iodine is too low, it may not destroy enough of the overactive follicular cells. Patients who are treated with doses of 131-Iodine that are too low may continue to demonstrate evidence of persistent hyperthyroidism. If a dose of 131-Iodine is higher, it may destroy much of the remainder of the thyroid gland in addition to the overactive follicular cells. Patients who are treated with higher doses of 131-Iodine may eventually develop a condition called postablative hypothyroidism that is associated with underactivity of the thyroid gland. It would seem that an ideal plan would be to choose a dose of 131-Iodine that is high enough to destroy a sufficient number of the overactive follicular cells without destroying the remainder of the thyroid gland. In practice, it is often difficult to choose a dose of 131-Iodine that is this precise. As a rule, Graves’ disease tends to be an erratic disorder that follows an unpredictable course. This makes it difficult to rely on specific calculations to choose precisely targeted doses of 131-Iodine. In light of this, some doctors may intentionally choose to administer higher doses of 131-Iodine. This approach is based on the rationale that it may be more desirable to provide a single course of definitive treatment that leads to the development of postablative hypothyroidism than to risk the possibility of ineffective treatment.

Doses of 131-Iodine that are used to treat patients with Graves’ disease typically range from 5-15 mCi (millicuries) per dose. A millicurie is a unit used to measure the amount of energy released by a radioactive isotope. In practice, doses of 131-Iodine may be calculated by using a formula that takes into account the estimated size of a patient’s enlarged thyroid gland along with the level of activity measured on a thyroid uptake study. A patient who presents with markedly enlarged thyroid gland may require a higher dose of 131-Iodine to provide adequate distribution of radioactive iodine throughout the thyroid gland. A patient who presents with a higher level of activity may require a lower dose of 131-Iodine, as more of the radioactive iodine administered will be taken up and stored in the thyroid gland. Some doctors may not rely on formulas to calculate doses, choosing instead to administer a fixed dose based on the severity of a patient’s hyperthyroidism.