Thiamazole Other names: Methimazole Tapazole

In general, pregnancy has a positive effect on hyperthyroidism. Nevertheless, treatment of hyperthyroidism is often required especially in the first months of pregnancy. Untreated hyperthyroidism during pregnancy may lead to serious complications such as 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 foetal blood, reaches concentrations equal to those found in maternal serum. At an inappropriate dosage, this may lead to goitre formation and hypothyroidism in the foetus 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 first weeks of pregnancy, e.g. choanalatresia, oesophageal atresia, hypoplastic nipples, delayed mental as well as motor development. In contrast, several case studies on prenatal thiamazole exposition have neither revealed any morphological development disorders nor affection of the thyroid or the physical and intellectual development of the children.

Since embryotoxic effects cannot be completely excluded, thiamazole must only be administered during pregnancy after strict benefit risk evaluation 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.

Breast-feeding is possible during thiamazole treatment; however, only low doses up to 10 mg daily may be used without additional administration of thyroid hormones.

The function of the thyroid gland of the neonate has to be monitored regularly.

Thiamazole has no influence on the ability to drive and use machines.