Context: Microchimerism is the presence of small populations of cells from one individual in another genetically distinct individual. This phenomenon can arise from pregnancy, blood transfusion, or bidirectional cell trafficking between twins in utero. Microchimerism has recently been proposed to play a role in the pathogenesis of thyroid autoimmunity. In that case, twins from opposite-sex pairs (OS) should have an increased risk of thyroid autoantibodies (TA).

Aim: The aim of the study was to compare the frequency of TA in twin individuals from OS and monozygotic (MZ) twin pairs.

Design: This was a case-control study of 240 individuals (120 females and 120 males) from OS twin pairs (cases) and 568 control individuals from MZ pairs (284 females and 284 males).

Methods: Antibodies toward thyroid peroxidase (TPOAb), thyroglobulin (TgAb), and the TSH receptor (TSHRAb) were measured and considered positive if greater than 60 U/ml, greater than 60 U/ml, and greater than 1.0 U/liter, respectively.

Results: The frequency of TPOAb, TgAb, and TSHRAb among female cases was 15.0, 5.0, and 4.2%, respectively, which was higher than the corresponding prevalences in the female control population: 7.4% (P = 0.018), 1.1% (P = 0.023), and 0.7% (P = 0.026), respectively. However, when corrected for the number of phenotypes studied (TPOAb, TgAb, TSHRAb, and any thyroid antibody), the association remained significant only in the combined group, P_corrected = 0.012. Essentially similar results were obtained in males.

Conclusion: Both female and male twins from OS pairs, as opposed to MZ pairs, have an increased frequency of TA, indicating a potential role of microchimerism in developing TA.