Patients with Graves disease may be receiving unnecessarily high doses of radiation therapy, scientists warn.
Patients with Graves disease can feel extremely hot
Researchers from the University of Malmo, Sweden, writing in New Scientist say the treatment could increase a patient's risk of developing cancer.
But experts in the UK say numerous studies have failed to find an increased cancer risk.
Thousands of women in the UK have Graves disease, in which the thyroid gland produces too much of a hormone.
It is treated by destroying all or part of the thyroid using radioactive form of iodine, called iodine-131.
The therapy is not tailored to the individual patient, and a fixed dose of 370 megabecquerels is usually given, whatever the size of the thyroid gland or its effectiveness in processing iodine.
Helene Jonsson, of the Swedish Radiation Protection Authority, said that ignored guidelines from the International Commission on Radiological Protection, which say radiation exposure should be kept "as low as reasonably possible".
Ms Jonsson analysed 187 cases of Graves disease treated at Malmo Hospital between 1984 and 1988 where doses of iodine-131 were individually tailored.
She estimated some patients would have received up to two-and-a-half times as much radiation as they needed if they have been given the fixed dose.
Ms Jonsson said there was particular concern over the use of iodine-131 in children.
Professor Jane Franklyn of the University of Birmingham, has herself carried out research into whether doses can be tailored to individual patients.
She told BBC News Online: "There have been numerous studies which have attempted to get the dosage of iodine right for each patient. But we can't do it.
"Most experts do use a fixed dose in the expectation that it will cure the majority of patients."
She added: "There is no evidence there is any increased cancer risk."
Dan Ash, president of the Royal College of Radiologists, told New Scientist: "It is likely that some patients will get more than they need.
"But that is better than failing to treat the disease, especially when there
is no proof that the treatment causes long-term damage to health."