title: Antithyroid Drug Use in Pregnancy and Birth Defects: Why Some Studies Find Clear Associations, and Some Studies Report None.
abstract: BACKGROUND
  Rare cases of birth defects after the use of methimazole (MMI) or carbimazole to treat hyperthyroidism in early pregnancy have been reported since 1972, whereas propylthiouracil (PTU) has not been considered teratogenic. Recently, two studies reported birth defects after the use of MMI in early pregnancy to affect 2-4% of exposed children, and one study also found birth defects after the use of PTU. On the other hand, some published studies did not find associations between the use of thionamides and birth defects.
  
  
  CONCLUSIONS
  The methods used in the two positive and the four negative reports are reviewed. The two positive studies included a sufficient number of children exposed to MMI (n = 1231 and 1097) to evaluate the studied outcomes, whereas the four negative studies included a much lower number of exposed children (n = 73, 108, 30, and 124). Considering PTU, the birth defects observed in one study were in general milder and tended to be diagnosed and registered only when they resulted in complications and led to surgery after one year of age. None of the negative studies has investigated outcomes after one year of age.
  
  
  CONCLUSIONS
  Studies finding no associations between early pregnancy exposure to antithyroid drugs and birth defects were either not sufficiently powered or did not study outcomes at optimal ages.
authors.affiliation: 1 Department of Endocrinology, Aalborg University Hospital , Aalborg, Denmark .|2 Department of Clinical Medicine, Aalborg University Hospital , Aalborg, Denmark .
authors.forename: Peter
authors.initials: P
authors.lastname: Laurberg
authors.affiliation: 1 Department of Endocrinology, Aalborg University Hospital , Aalborg, Denmark .|3 Department of Clinical Biochemistry, Aalborg University Hospital , Aalborg, Denmark .
authors.forename: Stine Linding
authors.initials: SL
authors.lastname: Andersen
mesh_terms: D000014:Abnormalities, Drug-Induced / Q000175:diagnosis / Q000209:etiology*; D013956:Antithyroid Agents / Q000009:adverse effects* / Q000627:therapeutic use; D005260:Female; D006801:Humans; D006980:Hyperthyroidism / Q000188:drug therapy; D011247:Pregnancy; D011248:Pregnancy Complications / Q000188:drug therapy
chemical_list: D013956:Antithyroid Agents