Pregnancy and Thyroid Disease

Pregnancy and Thyroid Disease

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 outcomes1

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.


Thyroid disease can cause various problems for a woman who wants to get pregnant:

  • thyroid problems can cause women not to ovulate so it may be difficult to conceive if you have undiagnosed or undertreated hypothyroidism1
  • hypothyroidism and subclinical hypothyroidism can cause an increase in prolactin, the hormone that induces and maintains the production of breast milk. It is not clear whether high prolactin (hyperprolactinaemia) has a negative effect on fertility but if you have high prolactin levels, do get your thyroid function checked2
  • there is increasing evidence that autoantibodies cause subfertility and early pregnancy loss, even in euthyroid women1,3
  • undiagnosed hyperthyroidism can cause miscarriages, premature births and problems for the baby1

NICE state that all women who have overt hypothyroidism or subclinical hypothyroidism and who are planning a pregnancy or who are pregnant should have a referral to an endocrinology specialist.4  Your doctor should discuss initiation of treatment and dosage with an endocrinologist whilst waiting for the referral.

NICE also state that thyroid function tests should be checked before conception if possible and that if they are not within the euthyroid range, they should be advised to delay conception and use contraception until the woman is stabilised on levothyroxine.4

There have been several studies of pregnant women with high TPO antibodies which found that their children had impaired intellectual performance whether or not the mothers also had experienced clinical thyroid dysfunction so you need to ensure that your doctor tests for these before trying for a baby.5

Your doctor should also explain that you are likely to need more levothyroxine during pregnancy and that your dosage should be adjusted as early as possible in the pregnancy to reduce the chance of problems for you and your baby.


In the first trimester of pregnancy the foetus is dependent on maternal thyroid hormones for normal development of the brain; hence it is vitally important that adequate maternal levels of thyroid hormones are maintained.6

This means that during the first 12 weeks of pregnancy, the baby uses the mother’s thyroid hormone because its own thyroid gland does not become active until 12 weeks although it doesn’t make enough thyroid hormone for its needs until around 18-20 weeks.6

So, if the mother has hypothyroidism and relies on levothyroxine, she may not have enough for herself and her baby.

If the baby does not get enough thyroxine from the mother this can cause a number of problems such as miscarriage and low IQ in infants.6

Research into whether women with subclinical hypothyroidism should be treated remains inconclusive.7

If you are hypothyroid do speak to your doctor or endocrinologist as soon as you suspect you are pregnant, or if it has already been confirmed, about changes to your dosage of levothyroxine or any other thyroid hormone replacement you are taking.  Also, make sure you discuss with your doctor a referral to a thyroid specialist.

If you have subclinical hypothyroidism and are not yet on any treatment, do speak to your doctor before you try for a baby to check your levels and if you become pregnant visit your doctor immediately to check your levels in case you become overtly hypothyroid and need treatment.

The British Thyroid Association Executive Committee state in their 2015 guidance, “Management of primary hypothyroidism” that, “The serum TSH reference range in pregnancy is 0.4–2.5 mU/l in the first trimester and 0.4–3.0 mU/l in the second and third trimesters or should be based on the trimester-specific reference range for the population if available.” 8

This is another reason you should get your levels checked as soon as you know you are pregnant.



Graves’ disease is associated with adverse pregnancy outcomes, including preterm delivery, pre‐eclampsia, growth restriction, heart failure and stillbirth.9

If you have recently had radioactive iodine therapy, you should wait for six months before trying to conceive.10

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.10

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.10

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.10

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.10

Even if a patient has previously been treated successfully for hyperthyroidism, it does not guarantee there will be no problems during the pregnancy.11 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.12

Check out what people are saying on our online community regarding pregnancy and thyroid disease:

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

2014 – 2014 European Thyroid Association Guidelines for the Management of Subclinical Hypothyroidism in Pregnancy and in Children

2014 – Guidelines for the Treatment of Hypothyroidism

2015 – Management of primary hypothyroidism: statement by the British Thyroid Association Executive Committee


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

2017 -Thyroid hormone treatment among pregnant women with subclinical hypothyroidism: US national assessment


Antibodies  – protein produced in the blood that fight diseases by attacking and killing harmful bacteria

Carbimazole  – the most commonly used medicine for hyperthyroidism. It works by reducing the amount of thyroid hormones which your thyroid gland makes

Endocrinologist – a medical practitioner qualified to diagnose and treat disorders of the endocrine glands and hormones

Goitre – a swelling of the neck resulting from enlargement of the thyroid gland

Graves’ disease – an immune system disorder that results in the overproduction of thyroid hormones (hyperthyroidism)

Hyperthyroidism – a term used to describe an over-active thyroid gland

Hypothyroidism – a term used to describe an under-active thyroid gland

Levothyroxine – a synthetic thyroid hormone commonly given to treat an underactive thyroid. It is also known as L-thyroxine

Neonatal  – of or for babies that were born recently

Ovulate – (of a woman or female animal) to produce an egg from which a baby can be formed

Pre-eclampsia  – a condition in which a pregnant woman or a woman who has just given birth has high blood pressure, swelling of parts of the body, and too much protein in the urine

Subclinical – not detectable, or producing effects that are not detectable, by the usual clinical tests

Thyrotoxicosis – the condition that occurs due to excessive thyroid hormone of any cause and therefore includes hyperthyroidism

Thyroxine – the main hormone secreted into the bloodstream by the thyroid gland. It is the inactive form


1. Thyroid dysfunction and reproductive health
The Obstetrician and Gynaecologist
Amanda Jefferys BMBS BMedSci MRCOG, Mark Vanderpump MBChB MD FRCP, Ephia Yasmin MBBS MD MRCOG

2. Hyperprolactinemia in association with subclinical hypothyroidism

3. Recurrent pregnancy loss in patients with thyroid function

Scenario: Preconception or pregnant!scenario:2

5. Childhood IQ, hearing loss, and maternal thyroid autoimmunity in the Baltimore Collaborative Perinatal Project

6. Thyroid Disease and Pregnancy

7. Subclinical Hypothyroidism in Women Planning Conception and During Pregnancy: Who Should Be Treated and How

8. Management of primary hypothyroidism: statement by the British Thyroid Association Executive Committee

9. Thyroid physiology and common diseases in pregnancy: review of literature

10. 2017 Guidelines of the American Thyroid Association for the Diagnosis and
Management of Thyroid Disease During Pregnancy and the Postpartum

11. The Effect of Pregnancy on Subsequent Relapse from Graves’ Disease after a Successful Course of Antithyroid Drug Therapy

12. Fetal neonatal hyperthyroidism: diagnostic and therapeutic approachment

Date Updated: 12/03/20 (V1.0)
Review date:   12/03/22