Thyroid Research Thyroid Research Archive Hyperthyroidism

No overall increase in major malformations in infants of mothers treated with methimazole during pregnancy

(March 2002)

The background of the study. Congenital malformations are rare in infants of mothers who received an antithyroid drug during the first months of their pregnancies, but they may be more common than in unexposed infants. This study assessed the risk of anomalies in infants of women taking methimazole during their pregnancies.

How the study was done. The patient group consisted of 288 pregnant women with hyperthyroidism who took methimazole during the first trimester of pregnancy. The control group consisted of 1089 pregnant women taking drugs known not to be teratogenic. Several months after the expected date of delivery the women were queried about the outcome of the pregnancy and presence and type of major malformations in the infants.

The results of the study. Information about the outcome of pregnancy was obtained from 241 (84 percent) of the women with hyperthyroidism who had taken methimazole. Among them, 200 delivered 202 live infants. Among the control women, 993 delivered 1002 live infants. The frequency of spontaneous abortions in the two groups was 6 percent and 7 percent, respectively. The age of the mothers, gestational age at delivery, and the weight and head circumference of the infants in the two groups were similar.

Among the infants exposed to methimazole, 8 (4 percent) had one or more major malformations: 1 each had esophageal atresia, choanal atresia, craniosynostosis and hypospadias, scrotal hypospadias, scrotal hypospadias and hemivertebra, spina bifida, ventricular septal defect, and atrioventricular canal. Among the infants in the control group, 23 (2 percent, P = 0.19) had major malformations: 8 cardiac, 6 renal, 3 skeletal, 2 hypospadias, 1 diaphragmatic hernia, 1 ocular, 1 cystic hygroma, and 1 choroid plexus cyst.

The conclusions of the study. The frequency of major malformations is not increased in infants of mothers with hyperthyroidism who took methimazole during pregnancy, as compared with infants whose mothers took non-teratogenic drugs.

The original article. Di Gianantonio E, Schaefer C, Mastroiacovo PP, Cournot MP, Benedicenti F, Reuvers M, Occupati B, Robert E, Bellemin B, Addis A, Arnon J, Clementi M. Adverse effects of prenatal methimazole exposure. Teratology 2001;64:262-6.