Individuals with type 1 diabetes have an increased risk for thyroid cancer without increased morbidity or mortality, according to study findings published in the Journal of the Endocrine Society.

Incidence of thyroid cancer diagnoses has been increasing globally, which may be attributed to overdiagnosis due to improved detection methods. Although previous research findings establish a higher prevalence of thyroid autoimmunity among individuals with type 1 diabetes, findings regarding thyroid cancer risk among this population remain unclear.

To ascertain whether adults with long-term type 1 diabetes have a true excess thyroid cancer risk, researchers sourced data from the Finnish Diabetic Nephropathy Study (FinnDiane) cohort, an ongoing, nationwide, multicenter study aimed at identifying genetic and environmental risk factors for complications of diabetes.

Participants were adults with type 1 diabetes, whereas control participants were selected from the Population Register Center, with each FinnDiane participant matched with up to 3 individuals according to sex, age, and place of residence.

The researchers identified thyroid cancers through the Finnish nationwide health registries and reviewed medical records to determine cancer type, presence of metastases, treatment, and recovery.

The study included a total of 4758 participants with type 1 diabetes and 12,710 control participants. Among the type 1 diabetes group, 4731 participants had no thyroid cancer and 27 participants had thyroid cancer. The thyroid cancer (mean age, 47.3 years) vs no thyroid cancer (mean age, 37.8 years) were 57.1% vs 47.5% women.

The rates of thyroid cancer between the type 1 diabetes and control groups were 0.57% vs 0.21% (standardized incidence ratio [SIR], 2.43; 95% CI, 1.59-3.56). Thyroid cancer incidence was higher among men (SIR, 3.88; 95% CI, 1.97-6.92) compared with women (SIR, 1.92; 95% CI, 1.09-3.14). However, men and women with type 1 diabetes had an equal relative risk for thyroid cancer (relative risk, 1.61; 95% CI, 0.75-3.45).

The median age at diagnosis of thyroid cancer was relatively young among individuals with type 1 diabetes (median age, 38.2 years) compared with the control group (median age, 42.8 years). The researchers noted 8 deaths among individuals with thyroid cancer, but these deaths were not due to cancer.

The rates of papillary thyroid cancer were 81.5% and 88.9% among the type 1 diabetes and control groups, respectively. The rest of the diagnoses were follicular thyroid cancer. The researchers identified no thyroid cancers with metastatic or invasive characteristics.

Among patients with type 1 diabetes, the rate of hypothyroidism was similar between those with and without thyroid cancer (18.5% vs 18.1%), whereas hyperthyroidism was diagnosed more often in those with vs without thyroid cancer (11.1% vs 2.3%; P =.003).

Study limitations include its retrospective nature and a lack of data on the relationship between thyroid cancer and autoimmune disease.

“Our results show that although there is an excess risk of thyroid cancer, it is only marginally increased in individuals with [type 1 diabetes] compared to control individuals and was not associated with increased morbidity or mortality,” the researchers concluded. “An overdiagnosis effect due to regular health care contacts is the most likely explanation for the higher risk.”

Disclosure: Multiple study authors declared affiliations with biotech, pharmaceutical, and/or device companies. Please see the original reference for a full list of authors’ disclosures.