Purpose: Thyroid autoimmunity has been associated with differentiated thyroid cancer although multiple potential biases might have influenced the results of previous studies.
Methods: We conducted a case-control study nested within the cohort of US active-duty personnel 1996-2014 to assess the association between thyroid autoimmunity, defined by serology, and thyroid cancer diagnosis. The primary exposure was thyroid peroxidase (TPO) antibody status 7-10 years before the thyroid cancer index date. We also assessed whether diagnosis of thyroid autoimmunity mediated any associations identified and if thyroid cancer features differed by autoimmunity status.
Results: Among 451 incident cases of papillary thyroid cancer and matched controls (median age 36 years, 61.4% men), TPO antibody positivity (v negative) 7-10 years prediagnosis was associated with thyroid cancer (odds ratio [OR] 1.90 [95% CI, 1.33 to 2.70]). Exploratory analyses suggested an increasing risk of thyroid cancer with higher TPO antibody titer (TPO antibody 550-1,399 IU/mL: OR 2.95 [95% CI, 1.37 to 6.36]; and ≥ 1,400 IU/mL: OR 3.91 [95% CI, 1.66 to 9.24]). Positive TPO antibody status remained associated with thyroid cancer after those with diagnosed autoimmunity were excluded, and the association was not mediated by diagnosis of thyroid autoimmunity. Among the cases with diagnosed autoimmunity, 58% thyroid cancers were ≤ 10 mm diameter.
Conclusion: Longstanding prior thyroid autoimmunity up to 10 years before thyroid cancer diagnosis was associated with papillary thyroid cancer risk. The results could not be fully explained by diagnosis of thyroid autoimmunity although when autoimmunity had been identified, thyroid cancers were diagnosed at a very early stage.