Methimazole is a medication used to manage an overactive thyroid gland, a condition known as hyperthyroidism. For pregnant individuals, concerns about its safety are common. Its use during pregnancy requires balancing maternal benefits with potential effects on the developing baby.
Why Methimazole Is Used During Pregnancy
Hyperthyroidism occurs when the thyroid gland produces too much thyroid hormone, accelerating the body’s metabolism. Careful management is crucial during pregnancy, as untreated hyperthyroidism can lead to serious complications for both the mother and the fetus. For the mother, these risks include preeclampsia, heart failure, and thyroid storm, a severe and life-threatening exacerbation of hyperthyroidism.
For the developing baby, uncontrolled maternal hyperthyroidism can result in low birth weight, premature birth, and even stillbirth. High levels of maternal antibodies, particularly in Graves’ disease, can also cross the placenta and cause hyperthyroidism in the fetus. Methimazole is prescribed to regulate thyroid hormone levels, mitigating these risks and supporting a healthier pregnancy.
Potential Risks of Methimazole for the Baby
Methimazole crosses the placenta and is associated with potential fetal harm, especially during the first trimester. A pattern of rare congenital malformations, known as Methimazole embryopathy, is linked to exposure. These include aplasia cutis congenita (congenital scalp defects) and choanal atresia (narrowing or blockage of nasal passages).
Other reported malformations include esophageal atresia, and various renal, skull, cardiovascular, gastrointestinal, and umbilical defects. The risk of these birth defects is low with clinically applied doses, but not entirely excluded. The lowest effective dose is recommended to minimize these risks, as higher doses increase the chance of adverse effects.
Managing Hyperthyroidism During Pregnancy
Managing hyperthyroidism during pregnancy requires careful medication choices and ongoing monitoring. Propylthiouracil (PTU) is recommended for the first trimester due to Methimazole’s association with congenital malformations during this early developmental period. PTU has a lower rate of placental transfer and less severe teratogenic effects compared to Methimazole.
By the second trimester, once the baby’s organs have largely formed, a switch from PTU to Methimazole is often recommended. This change is due to the potential for severe hepatotoxicity (liver damage) associated with PTU, though both drugs are equally effective. Throughout pregnancy, close monitoring of thyroid hormone levels (free T4 and TSH) is important to ensure the mother remains euthyroid (thyroid hormone levels within a healthy range). In rare instances where medication is not suitable or effective, surgical removal of the thyroid gland (thyroidectomy) may be considered, typically in the second trimester.
Methimazole Use While Breastfeeding
Methimazole use while breastfeeding is considered safe for infants of mothers with postpartum hyperthyroidism. Although Methimazole transfers into breast milk, the amount is minimal and not clinically significant. Studies show that maternal Methimazole doses up to 20 mg per day do not negatively affect the thyroid function or intellectual development of breastfed infants.
To further minimize infant exposure, mothers can wait 3 to 4 hours after taking Methimazole before breastfeeding. Routine assessment of the infant’s serum thyroid function is not recommended by some professional organizations; however, monitoring for appropriate growth and development during routine pediatric check-ups is advised. If a mother is taking a Methimazole dose above 30 mg, periodic monitoring of the infant’s thyroid function may be advisable, or if the baby shows any clinical symptoms.