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Hyperthyroid and Pregnant

Thyroid UK receives a lot of correspondence from women who are pregnant and not sure what to do to preserve the health of their baby.

There has been a lot of research over the past few years in regard to pregnancy and thyroid disease which has led to more updated guidelines on this.

Hyperthyroidism and hypothyroidism can both result in problems with conception, increased early miscarriage and adverse pregnancy and neonatal outcomes.1

Whether you are thinking about getting pregnant; are pregnant or are friends or family of someone who is pregnant, it’s important for you to know that untreated hypothyroidism, undertreated hypothyroidism and hyperthyroidism in pregnancy is bad for the baby.

Graves’ disease is associated with adverse pregnancy outcomes, including preterm delivery, pre‐eclampsia, growth restriction, heart failure and stillbirth.2

If you have recently had radioactive iodine therapy, you should wait for six months before trying to conceive.3

If you are hyperthyroid, on treatment for this and planning a pregnancy, you should always visit your doctor to ensure that your levels are within the range before you try to conceive.1

If you are receiving antithyroid medication when you become pregnant if you appear to be in remission,  your doctor may take you off this medication and do thyroid function tests during the first trimester to keep an eye on your levels. This is because antithyroid drugs can cross the placenta.  This needs to happen before 6 weeks though so it is important that you visit your doctor as soon as you know you are pregnant.3

If, however, you are not in remission and treatment needs to be continued,  your treatment may need to be changed.  Most people with hyperthyroidism are given carbimazole.  However, propylthiouracil  (PTU) is preferred during pregnancy because the risk for severe birth defects is lower. Whichever medication is given, your doctor should keep the dosage as low as possible as long as you are euthyroid.3

Patients on a “block and replace” regime, (antithyroid drugs and levothyroxine) should be taken off immediately (as this can cause foetal goitre and hypothyroidism) and only in rare circumstances should be allowed to continue.3

If you are on antithyroid drugs and want to breastfeed after the baby is born, experts have confirmed the safety of low to moderate doses of both PTU and carbimazole in breastfeeding babies.3

Even if a patient has previously been treated successfully for hyperthyroidism, it does not guarantee there will be no problems during the pregnancy.4 It’s possible that the patient will have a relapse after delivery. It’s possible that the baby could develop thyrotoxicosis before or after delivery if the mother has high antibodies.5


2012 – Management of Thyroid Dysfunction during Pregnancy and Postpartum: An Endocrine Society Clinical Practice Guideline

2017 –  Guidelines of the American Thyroid Association for the diagnosis and management of thyroid disease during pregnancy and the postpartum

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  1. Jefferys A, Vanderpump M, Yasmin E. Thyroid dysfunction and reproductive health. Obstet Gynecol. Published online January 2015:39-45. doi:10.1111/tog.12161
  2. Cignini P, Cafà E, Giorlandino C, Capriglione S, Spata A, Dugo N. Thyroid physiology and common diseases in pregnancy: review of literature. J Prenat Med. 2012;6(4):64-71.
  3. Alexander EK, Pearce EN, Brent GA, et al. 2017 Guidelines of the American Thyroid Association for the Diagnosis and Management of Thyroid Disease During Pregnancy and the Postpartum. Thyroid. Published online March 2017:315-389. doi:10.1089/thy.2016.0457
  4. Rotondi M, Cappelli C, Pirali B, et al. The Effect of Pregnancy on Subsequent Relapse from Graves’ Disease after a Successful Course of Antithyroid Drug Therapy. The Journal of Clinical Endocrinology & Metabolism. Published online October 1, 2008:3985-3988. doi:10.1210/jc.2008-0966
  5. Kurtoglu S, Ozdemir A. Fetal neonatal hyperthyroidism: diagnostic and therapeutic approachment. Turk Pediatri Ars. Published online March 14, 2017:1-9. doi:10.5152/turkpediatriars.2017.2513

Date updated: 04.05.21 (V1.1)
Review date: 12.03.22

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