Low-Dose Radioactive Iodine Destroys Thyroid Tissue Left after Surgery

Adapted from the NCI Cancer Bulletin.

A low dose of radioactive iodine given after surgery for thyroid cancer destroyed (ablated) residual thyroid tissue as effectively as a higher dose, with fewer side effects and less exposure to radiation, according to two European randomized controlled trials published May 3, 2012, in the New England Journal of Medicine. Both trials also showed that either administering thyrotropin alfa or withdrawing thyroid hormones before administering radioactive iodine was effective for thyroid ablation at both radioactive iodine dose levels.

Researchers from France and the United Kingdom enrolled 752 and 438 patients with low-risk thyroid cancer, respectively, in their trials (here and here). Both studies randomly assigned patients to receive either low-dose radioactive iodine (1.1 GBq) or high-dose radioactive iodine (3.7 GBq) several months after surgery.

For radioactive iodine treatment to work, thyroid-stimulating hormone (thyrotropin) levels must be elevated. The researchers therefore randomly assigned study participants in both dose groups to either withdrawal of the replacement thyroid hormones given after surgery (to allow the body’s own thyrotropin level to rise) or administration of recombinant thyrotropin (thyrotropin alfa). Although thyrotropin alfa causes less discomfort than the withdrawal of replacement thyroid hormones, there was some concern that the drug might interfere with the effectiveness of radioactive iodine treatment.

Low-dose and high-dose radioactive iodine resulted in similar numbers of successful ablations in both trials: about 95 percent of patients in the French trial and 85 percent of patients in the U.K. trial, which enrolled patients with larger tumors than the French trial. In both trials, at both dose levels, rates of successful ablation did not differ significantly between patients receiving thyrotropin alfa or those undergoing replacement thyroid hormone withdrawal.

Ablation of residual thyroid tissue with radioactive iodine makes it easier to monitor patients with low-risk disease for local or distant recurrence, and its use has been rising in the United States. One study found that the use of ablation in younger patients with low-risk thyroid cancer rose from about 3 percent in 1973 to about 40 percent in 2007.

But whether radioactive iodine improves survival for low-risk patients remains unknown. “That benefit is much harder to prove in low-risk individuals because their survival from this disease is so good to begin with,” explained Erik Alexander, M.D., of Harvard Medical School, who co-authored an editorial accompanying the two trials. A trial testing whether radioactive iodine improves disease-free survival in low-risk patients was recently launched in the United Kingdom.