A new study provides more evidence that people with thyroid autoimmunity are more likely than are others to develop papillary thyroid cancer (odds ratio [OR] = 1.90, 95% confidence interval [CI], 1.33-2.70), although the overall risk remains very low. Researchers aren't recommending routine screening in all patients with thyroid autoimmunity, but they're calling for more research into whether it's a good idea in severe cases. "This is the one circumstance where screening for subclinical disease could make sense," said Donald McLeod, MPH, PhD, an epidemiologist at Royal Brisbane & Women's Hospital in Australia and lead author of the study, published in the Journal of Clinical Oncology. "However, more research is needed because our study is the first to show this result, and we need to prove that screening would make a difference to the prognosis of these patients." According to Dr. McLeod, "doctors and patients have been wondering about the connection between thyroid autoimmunity and thyroid cancer for many years. In fact, the first report was in 1955. While the association was plausible, all previous studies had potential for biases that could have influenced the results." For example, he said, multiple studies didn't control for confounders, while others didn't account for the possibility that cancer could have triggered an immune response. "Other case-control studies could have been affected by selection bias, where a diagnosis of thyroid autoimmunity leads to thyroid cancer identification and entry into the study," he said. "Finally, medical surveillance of people diagnosed with thyroid autoimmunity could lead to overdiagnosis, where small, subclinical cancers are diagnosed in those patients but not identified in people who are not under medical follow-up." For the new retrospective case-control study, researchers compared 451 active-duty members of the U.S. military who developed papillary thyroid cancer from the period of 1996-2014 to matched controls (61% of all subjects were men and the mean age was 36). Those with cancer had their serum collected 3-5 years and 7-10 years before the date of diagnosis - the index date for all subjects. Some of those considered to have thyroid autoimmunity had conditions such as Graves' disease and Hashimoto's thyroiditis. "Eighty-five percent of cases (379 of 451) had a thyroid-related diagnosis recorded ... before their index date, compared with 5% of controls," the researchers reported. "Most cases (80%) had classical papillary thyroid cancer, with the rest having the follicular variant of papillary thyroid cancer." After adjustment to account for various confounders, those who were positive for thyroid peroxidase antibodies 7-10 years prior to the index date were more likely to have developed thyroid cancer (OR = 1.90, 95% CI, 1.33-2.70). "The results could not be fully explained by diagnosis of thyroid autoimmunity," the researchers reported, "although when autoimmunity had been identified, thyroid cancers were diagnosed at a very early stage." Two groups - those with the highest thyroid antibody levels and women - faced the greatest risk, Dr. McLeod said. The results regarding women were the most surprising in the study, he said. "This is the first time this has been found. We think this result needs to be confirmed. If true, it could explain why women have a three-times-higher risk of thyroid cancer than men." The overall incidence of thyroid cancer in the U.S. was estimated at 13.49 per 100,000 person-years in 2018, with women (76% of cases) and Whites (81%) accounting for the majority. Rates have nearly doubled since 2000. The authors of a 2022 report that disclosed these numbers suggest the rise is due to overdiagnosis of small tumors. It's not clear why thyroid autoimmunity and thyroid cancer may be linked. "Chronic inflammation from thyroid autoimmunity could cause thyroid cancer, as chronic inflammation in other organs precedes cancers at those sites," Dr. McLeod said. "Alternatively, thyroid autoimmunity could appear to be associated with thyroid cancer because of biases inherent in previous studies, including previous diagnosis of autoimmunity. Thyroid cancer could also induce an immune response, which mimics thyroid autoimmunity and could bias assessment." As for screening of patients with thyroid autoimmunity, "the main danger is that you will commonly identify small thyroid cancers that would never become clinically apparent," he said. "This leads to unnecessary treatments that can cause complications and give people a cancer label, which can also cause harm. Diagnosis and treatment guidelines recommend against screening the general population for this reason." Many of those with thyroid autoimmunity developed small cancers, he said, most likely "detected from ultrasound being performed because autoimmune thyroid disease was known. If all patients with thyroid autoimmunity were screened for thyroid cancer, the likelihood is that many people's cancers would be overdiagnosed." The study was funded by the Walton Family Foundation. Dr. McLeod reports no disclosures. Some of the authors report various relationships with industry.
