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Pregnancy and hyperthyroidism

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What complications could occur if hyperthyroidism
is not controlled during pregnancy?


Pregnancy may be affected by hyperthyroidism (an overactive thyroid gland). If possible, hyperthyroidism should be treated and brought under control before getting pregnant. If hyperthyroidism is uncontrolled, there is a high risk of complications of pregnancy. For the baby, these include miscarriage, premature delivery (when the baby is born too soon), low birth weight and abnormal development. For the mother, these include an increased risk of severe complications of pregnancy, affecting her health and putting her baby at risk.

It is very important that you inform your GP and any other doctors looking after you if you are being treated for hyperthyroidism and are pregnant or trying for a baby, as your medication needs careful monitoring and may need adjusting.


What happens to the mother’s thyroid disease in pregnancy?


Sometimes hyperthyroidism in the mother gets better late on in pregnancy, so that her treatment can be reduced. The thyroid gland often becomes overactive again after the birth of the baby. This tends to happen 3-4 months after the birth.


Will my thyroid disease affect my baby?


Hyperthyroidism can affect the baby in the womb or the newborn baby. This happens in 1-5% of women with thyroid desease during pregnancy (1-5 per 100 women). It is tested for when the mother is about six months pregnant, so that it can be monitored and treated. If the mother is taking tablets for the thyroid, the baby’s thyroid should also be controlled. If the baby develops an overactive thyroid gland after birth, this can be treated and usually gets bettter by itself.


How is hyperthyroidism treated during pregnancy?


Tablet treatment is preferred and is described further in seperate leaflets. Either carbimazole or propylthiouracil (PTU) may be used, at the lowest dose that controls the overactive thyroid. Mothers on low doses of PTU or carbimazole can safely breastfeed their baby.


Are there side-effects to the baby of tablet treatment during pregnancy?


Tablet treatment is associated with a very small risk of developmental abnormalities of the baby (for example, undeveloped patches in the baby’s scalp): your obstetrician will discuss this with you in more detail. The risks to the baby of tablet treatment in pregnancy are much lower than the risks to the baby of uncontrolled hyperthyroidism in pregnancy.

Another option in women with an overactive thyroid gland who are planning to become pregnant is to have permanent treatment before the pregnancy, by surgery or radioiodine therapy. You should not try to conceive until 6 months after radioiodine therapy.

Surgery (a ‘thyroidectomy’ operation) can be performed in the middle third of pregnancy (although drug therapy is preferred). Radioiodine therapy is not used in pregnant or breastfeeding women.


Who do I contact if I have any questions?


Endocrine Specialist Nurses
uhsussex.ens@nhs.net

Royal Sussex County Hospital
Endocrine specialist nurse
01273 696955 extension 64379

Princess Royal Hospital, Haywards Heath
Endocrine specialist nurse
01444 441881 extension 65660

If you have any urgent or emergency questions then your GP can advise you or they will contact the hospital on your behalf.

Patient self-help groups and further information: British Thyroid Foundation



This information is intended for patients receiving care in Brighton & Hove or Haywards Heath. The information in this leaflet is for guidance purposes only and is in no way intended to replace professional clinical advice by a qualified practitioner.

Publication Date: July 2021

Review Date: April 2024

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