Thyroid function abnormalities and thyroid autoantibodies in Danish pregnant women

OBJECTIVE: Abnormal thyroid function in pregnant women is a matter of concern. Knowledge on the occurrence of known and unidentified thyroid function abnormalities in a large unselected cohort of pregnant women is warranted as part of the debate on benefits and risks of routine testing. DESIGN: Cohort study. PARTICIPANTS: A total of 14 323 pregnant women in the North Denmark Region, who had a blood sample drawn as part of the prenatal screening program in early pregnancy (2011-2015). MEASUREMENTS: TSH, free thyroxine, thyroid peroxidase and thyroglobulin antibodies were measured in the stored blood samples using an automatic immunoassay (ADVIA Centaur XPT, Siemens Healthineers). Cohort-, method- and week-specific reference ranges were used for classification of maternal thyroid function, and a cut-off of 60 U/mL was used for thyroid autoantibodies. Information in Danish nationwide registers was used to identify diagnosed and treated maternal thyroid disease. RESULTS: Overall, 15.2% had thyroid function abnormalities in the early pregnancy and 14.9% were thyroid peroxidase and/or thyroglobulin antibody positive. Among women with known thyroid disease (n = 365), the frequency of abnormal thyroid function was 45.7%, and 62.8% in women (n = 172) who received current treatment in the pregnancy. When maternal thyroid disease was diagnosed in the years following pregnancy (n = 313), 46.7% had abnormal thyroid function and 54.3% were thyroid peroxidase and/or thyroglobulin antibody positive in the early pregnancy. CONCLUSION: Thyroid function abnormalities and thyroid autoantibodies were common in Danish pregnant women, particularly in women with known or later diagnosed thyroid disease, which raises concern about inadequately treated and unidentified abnormal thyroid function.