<p>Pregnancy Category D. Due to the rare occurrence of congenital malformations associated with methimazole use, it may be appropriate to use other agents in pregnant women requiring treatment for hyperthyroidism particularly in the first trimester of pregnancy. Methimazole is excreted into breast milk. However, several studies found no effect on clinical status in nursing infants of mothers taking methimazole, particularly if thyroid function is monitored at frequent (weekly or biweekly) intervals.</p>