It's a curious finding because at the same time it's being used more and more, but are the right patients getting the right treatment is the question researchers are now left with.
About a year and a half ago Jennifer Anderson was a new mom and was also diagnosed with thyroid cancer.
She is one of some 45 thousand Americans who'll be found to have cancer of the thyroid this year.
The thyroid is the butterfly shaped gland that sits at the base of the throat.
Cancer there develops mostly in younger adults - people in their 30's, 40's and 50's.
"You hear the word cancer and you freak out a little bit like, 'Am I going to die? I just had this son. I need to be around for him," Jenifer Anderson said.
Treatment included surgery and then was also treated with radioactive iodine.
"For me, the iodine was definitely something that reassured me that it was killing any other cells there might have been," Anderson said.
But the use of the iodine treatment does not appear to be widely standardized.
"We found that over time there's been an increase in the use of radioactive iodine and we found that there's a wide variation in the use," Dr. Megan R. Haymart of the University of Michigan, Ann Arbor said.
Haymart studied the records of patients from nearly one thousand U.S. hospitals over a period of nearly two decades. They were patients with both a high and low risk of recurrence.
She found that the treatment use varied widely.
"It isn't just your disease severity, but it's also where you get your care that makes a difference in what treatment you receive," Haymart said.
There could be some uncertainty about which patients should be treated, she says.
"There's wide variation in the treatment of thyroid cancer and so, for patients, I think it's important that they talk with their physicians that they understand their disease and the risk benefit of radioactive iodine," she said.
The study reports that clinical guidelines have left radioactive iodine use up a physician's discretion. There are four different kinds of thyroid cancer. Radioactive iodine is used in two of these and in some cases, the benefits may not exceed the risks, which is why conversations with your doctors -- and possibly second opinions at a different hospital -- may assure a patient they are getting the best care.