Information and support I am Living with a thyroid condition Managing hyperthyroidism in conception and pregnancy What is the difference between overt and subclinical hyperthyroidism? The main cause of hyperthyroidism is Graves’ disease. This is where the thyroid becomes overactive because of the presence of thyroid antibodies (TSH-receptor antibodies, TRAb). Overt hyperthyroidism is defined as having a TSH below the reference range and FT4 above the reference range. Subclinical hyperthyroidism is defined as having a TSH below the reference range but FT4 and FT3 within the reference range. This does not require treatment. What are the risks? If you have untreated or poorly controlled hyperthyroidism, your pregnancy may be at higher risk of certain complications, including pre-eclampsia, low birthweight and having a child with attention deficit hyperactivity disorder (ADHD) or autism. These risks can all be reduced if your thyroid levels are well-controlled before and during pregnancy. Before pregnancy You should talk to your doctor about these risks before you plan a pregnancy. Your doctor will also discuss the options of offering long-term treatments for your hyperthyroidism. Options include radioiodine treatment or removal of your thyroid gland. If you are treated with radioiodine you should wait at least 6 months before trying for a baby because the radiation may cause damage to your baby. If you have had thyroid surgery it is advisable that you have had 2 thyroid blood tests 3 months apart that show your blood tests are within the reference range. If your TSH-receptor antibody (TRAb) level remains raised at around 6 months after definitive treatment, you should consider delaying becoming pregnant. This is because raised TRAb levels can increase the risk of overactive thyroid disease in your unborn baby. If you are being treated with carbimazole when trying for a baby, this should be switched to propylthiouracil (PTU). This is because PTU is safer in conception and early pregnancy. Your doctor should aim for the lowest possible dose of antithyroid drugs. Your FT4 should be in the upper half of the reference range. Your care should be managed by an endocrinologist or obstetrician. During pregnancy If you are being treated with carbimazole this should be switched to propylthiouracil (PTU) before 10 weeks of pregnancy. Your doctor should aim for the lowest possible dose of antithyroid drugs. Your FT4 should be in the upper half of the reference range. Your thyroid function should be checked 2-4 weekly until 20 weeks. It should then be checked 4-8 weekly. Graves’ disease often gets better in pregnancy. If your antithyroid drugs are stopped, you may not need your thyroid function testing as frequently. If you have a history of Graves’ disease (even if you have had definitive treatment) you should have your TSH-Receptor antibodies (TRAb) tested at booking (12 weeks). Your TRAb should be tested again at 20 and 28 weeks if it is either 3 x above the TRAb level at 12 weeks, or you are on antithyroid drugs. It is recommended you have monthly ultrasound scans from 26 to 28 weeks if during pregnancy you have: Had uncontrolled Graves’ disease Required antithyroid drug treatment during pregnancy. Had a TRAb level 3 x above the normal level. After birth Your doctor will aim to get your thyroid function levels in the non-pregnancy reference ranges. Your thyroid function, including TRAb, should be checked 6 weeks after birth Both carbimazole and PTU are safe in breastfeeding. You may need to split your dose across the day. Read the RCOG Green-top guideline on the Management of Thyroid Disorders in Pregnancy Manage Cookie Preferences Please ensure Javascript is enabled for purposes of website accessibility