Prescribing in pregnancy. Thyroid disease.

When treating thyroid disease, as with other conditions in pregnancy, one is concerned with the welfare of both mother and developing child. Thyroid disease causes few maternal problems; thyrotoxicosis in fact tends to improve in pregnancy, allowing medical management with lower drug doses than usual. Relapse of thyroid disease may occur postpartum, when transient hypo- and hyperthyroidism are relatively common. In contrast, the fetus and neonate are threatened in a number of ways by drugs given to the mother and by transplacental passage of maternal antibodies capable of inducing thyroid disease. Antithyroid drugs may cause fetal goitre with airway obstruction, and are associated with mild neonatal hypothyroidism. Thyroid antibodies in primary myxoedema and Hashimoto's thyroiditis are occasionally implicated in neonatal hypothyroidism and may even cause thyroid dysgenesis. Neonatal hyperthyroidism has a high morbidity and mortality and may have long-term skeletal effects such as craniosynostosis. Fetal problems may not be apparent at birth but may emerge in the next eight to ten days, especially in hyperthyroidism when the mother has been on treatment. Close monitoring throughout pregnancy and for the first ten days postpartum is required to minimize risks to the fetus and neonate. Most pregnancies associated with thyroid disease will have a successful outcome. If the occasional at-risk fetus is to be identified and treated successfully there should ideally be close cooperation between obstetrician, endocrinologist and paediatrician.