Combination therapy with thiamazole and thyroid hormones is contraindicated during pregnancy.
In general, pregnancy has a positive effect on hyperthyroidism. Nevertheless, treatment of hyperthyroidism is often required especially in the 1st months of pregnancy. Untreated hyperthyroidism during pregnancy may lead to serious complications eg, premature birth and malformation. However, hypothyroidism caused by treatment with inappropriate thiamazole doses is also associated with a tendency to abortion.
Thiamazole passes the placental barrier, and in fetal blood, reaches concentrations equal to those found in maternal serum. At an inappropriate dosage, this may lead to goiter formation and hypothyroidism in the fetus as well as to reduced birth weight. There have been repeated reports of partial aplasia cutis on the head of neonates born to women treated with thiamazole. This defect healed spontaneously within a few weeks.
In addition, a certain pattern of diverse malformations has been associated with high-dose thiamazole therapy during the 1st weeks of pregnancy eg, choanalatresia, oesophageal atresia, hypoplastic nipples, delayed mental as well as motor development. In contrast, several case studies on prenatal thiamazole exposure have neither revealed any morphological development disorders nor impact on thyroid development or the physical and intellectual development of the children.
Since embryotoxic effects cannot be completely excluded, Thyrozol may only be administered during pregnancy after carefully weighing the benefit-risk ratio and only at the lowest still effective dose level without additional administration of thyroid hormones.
Thiamazole passes into breast milk where it can reach concentrations corresponding to maternal serum levels, so that there is a risk of hypothyroidism developing in the infant.
Breastfeeding is possible during thiamazole treatment; however, only low doses up to 10 mg daily may be used without additional administration of thyroid hormones. The infant's thyroid function is to be monitored regularly.