Hyperthyroidism - Treatment in Pregnancy
Doctors prefer to use propylthiouracil (PTU) during pregnancy, because it is considered a safer option than carbimazole. Carbimazole can cause a rare congenital abnormality of the scalp, called aplasia cutis. PTU however can cross the placenta and may cause hypothyroidism in the baby. Usually the lowest possible dose is given to avoid complications in the baby.
If you take PTU when you are pregnant then you must monitor your thyroid hormone levels every month. Thyroxine levels need to remain optimal (upper third of the normal reference range), because studies consistently confirm that low maternal free circulating thyroxine (not the maternal TSH), also referred to as hypothyroxinemia, can cause developmental delay or future neurodevelopmental difficulties in the baby.
PTU can control the symptoms within a few weeks. Both PTU and carbimazole can be excreted in the breast milk and it may not be safe for the baby. The evidence so far is inconclusive. PTU is considered safer during breastfeeding than carbimazole, because smaller amounts are excreted in the breast milk.
If hyperthyroidism cannot be controlled with low dose antithyroid medication, then surgery may be recommended. It is possible to have a thyroidectomy (removal of the thyroid gland) whilst pregnant. The safest time to do the surgery is in the second trimester. Antithyroid drugs are given prior to the surgery to avoid "thyroid storm".
Radioactive iodine should never be used in pregnancy, because it can cross the placenta and destroy the baby's thyroid gland.