An undiagnosed or uncontrolled thyroid disorder can make it harder to conceive and can cause problems during pregnancy. Julia Priestley writes about some of the common issues surrounding pregnancy and thyroid disorders.

Fertility problems

There are many reasons why couples might find it difficult to conceive and a poorly functioning thyroid is an often overlooked factor, particularly in female subfertility. According to Prof Simon Pearce, Consultant Endocrinologist, Newcastle upon Tyne ʻUndetected thyroid problems can cause significant problems with fertility but are generally straightforward to correct once identified.ʼ

The symptoms of a poorly functioning thyroid can often be subtle so it is important for women to see their GP if they have been trying to conceive for longer than a year to check their thyroid is functioning correctly before starting other medical procedures. This is particularly important if they have a history of thyroid disorders in their family as many are inherited.

For women, a normally functioning thyroid gland is essential to ovulation, implantation and maintenance of a healthy pregnancy. In addition, thyroid hormones, previously thought not to affect male fertility, are now being recognised as playing an important role. For example in men with hypothyroidism and hyperthyroidism, there may be a marked reduction in sperm count and sperm quality, and this may result in infertility. Fortunately the sperm abnormalities usually return to normal once the thyroid condition has been treated.

The most common type of thyroid disorder in child-bearing women is hypothyroidism. Low levels of thyroid hormone can interfere with the release of an egg from the ovaries (ovulation), which impairs fertility. In addition, some of the underlying causes of hypothyroidism, for instance, Hashimotoʼs thyroiditis, appear to also impair fertility.

Treating hypothyroidism in women is an important part of any effort to correct infertility: women who have hypothyroidism and hope to become pregnant, need to work with their doctor to make sure their hypothyroidism is under tight control. Women who have hypothyroidism and become pregnant, need to tell their doctor as soon as pregnancy is confirmed. Close monitoring of thyroid hormone levels during pregnancy can help normal fetal development and reduce the risk of miscarriage.

Pregnancy

If you are pregnant and have a history of thyroid disease (even if you are not on treatment now) for example

• subclinical hypothyroidism
• thyroiditis
• radioactive iodine treatment
• thyroidectomy
• goitre
• positive thyroid antibodies

Speak to your GP and arrange thyroid blood tests as soon as you have a positive pregnancy test.

Hypothyroidism

If you are planning a pregnancy you should speak to your GP to arrange thyroid blood tests and ideally aim for a thyroid-stimulating hormone (TSH) level of less than 2.5mU/l at the time of conception and in the first trimester of pregnancy, and less than 3.0mU/l after that.

As soon as you know you are pregnant it is generally recommended that your levothyroxine is increased immediately, usually by 25-50mcg daily. This is most easily achieved by doubling your current dose on two days of the week. You should then contact your GP and arrange to have a thyroid blood test. One exception to this however may be women who have had treatment for thyroid cancer and are already on doses of levothyroxine that keep their TSH level suppressed. These women will probably not need to increase their levothyroxine but should discuss the issue with their GP or specialist.

Thyroid blood tests should be checked every four to six weeks during pregnancy and with a further test a few weeks after delivery. But it may not be necessary to test so frequently in later pregnancy if the thyroid levels have been stable up until that point.

After delivery you will probably need to return to your pre-pregnancy dose of levothyroxine and patients taking levothyroxine for subclinical hypothyroidism may be able to stop treatment. You should discuss this with your GP or specialist before making any changes to your dose. Breastfeeding is safe while taking levothyroxine.

Hyperthyroidism

If you have an overactive thyroid you should discuss your plans for pregnancy with your endocrinologist before you try to conceive as it may be necessary to change your medication, or in some cases, it can be stopped. There is a small risk of birth defects with all antithyroid drugs during pregnancy, but taking these drugs is safer to both mother and baby than uncontrolled thyroid overactivity during pregnancy. The risk occurs because antithyroid drugs (e.g. carbimazole or prophylthiouracil (PTU)) cross the placenta so the lowest possible dose will be prescribed to ensure that the baby is less likely to be affected. In most cases PTU is the drug of choice when trying to conceive and in the first three months of pregnancy but you should discuss these issues with your endocrinologist before embarking on pregnancy.

If you are being treated with antithyroid drugs and you havenʼt already discussed your pregnancy plans with them beforehand, you should contact your endocrinologist or GP as soon as possible after pregnancy is confirmed as it may be necessary to adjust your medication.

You may need to have your thyroid blood tests checked every four weeks throughout the pregnancy but not all patients will need checks this frequently. Again, this will be decided by your endocrinologist.

If you are using antithyroid drugs and develop a rash, sore throat or an unexplained fever you must see a doctor immediately and ask for a full blood count to be arranged due to the small risk of agranulocytosis (low white blood cells).

People using PTU have a very small risk of liver problems so if you notice any itching or yellowing of your eyes you must see a doctor immediately.

Hyperthyroidism can recur during the first year after the baby is born so you should arrange to have your thyroid bloods checked around three months after delivery and at regular intervals after that. If you stopped taking antithyroid drugs during your pregnancy you should check with your doctor if you notice any symptoms of hyperthyroidism.

Only small amounts of antithyroid drugs cross into breast milk so you should be able to breastfeed provided the dose is small but check this first with your doctor. Antithyroid drugs are best taken in smaller doses over two or three times a day following a feed. If you plan to breastfeed for a long time your baby can have a blood test to check whether the thyroid is being affected.

The overriding message from medical professionals is that the earlier thyroid problems are acted on in pregnancy the better the outcome for the unborn child.

Can babies inherit thyroid disorders?

Congenital hypothyroidism (CHT) is rare in babies (about one in 2000-3000 in the UK) and in most cases the cause is unknown and there is no way to prevent it.

Thyroid gland development in a baby begins very early in pregnancy. The gland begins to form at the back of the tongue and moves to its normal position in the lower neck by eight weeks. In some babies the gland does not develop properly and/or may not move to the normal position. This form of CHT is called ʻdysgenesisʼ. The risk of having another child with this type of CHT is low.

In some cases, the thyroid gland develops and moves into the correct position but there is a problem with the thyroid hormone ʻproduction lineʼ and it has difficulty making thyroxine. This is called ʻdyshormonogenesisʼ. This type of CHT may occur if a baby has inherited a faulty CHT gene from mum and/or dad and there is a risk that the babyʼs siblings will also be affected. If this is suspected parents may be referred to a genetic counsellor who will be able to advise about risks in future pregnancies.

On rare occasions women with Gravesʼ antibodies can pass them through the placenta to the baby which may cause temporary hyperthyroidism in the baby during pregnancy and after birth but this is treatable.

Post-partum thyroiditis

This is usually a temporary disorder which occurs in women with positive thyroid autoantibodies. It usually shows up in the first six months after giving birth. The thyroid may be a little swollen but it is rarely painful. It usually starts with symptoms of an overactive thyroid and may resolve by itself, or progress to symptoms of an underactive thyroid.

Postpartum thyroiditis can be diagnosed by a physical examination and blood tests. It is important to distinguish hyperthyroidism due to thyroiditis from that caused by Gravesʼ disease. A test for thyroid antibodies will help determine this.

In mild cases there may be no need for treatment as it may clear up quickly. If the hyperthyroid symptoms are difficult to manage you may be prescribed a betablocker to relieve them. Levothyroxine tablets may be prescribed to manage hypothyroid symptoms.

Most women will make a full recovery from postpartum thyroiditis. If, however, blood tests reveal abnormalities more than a year after the birth, it is unlikely that these changes will resolve of their own accord and you will need further treatment.

This condition often returns after subsequent pregnancies so it is important to have your thyroid tested after each birth, usually after two to three months. Because postpartum thyroiditis increases the risk of permanent thyroid disease in the future it is advisable to have regular blood tests once a year to check thyroid hormone levels.

BTF guidance for pregnancy and thyroid disorders  

We have produced some guidance for pregnancy and thyroid disorders. This includes a pocket-sized pregnancy alert card that sets out the key messages for patients with thyroid disorders who are planning pregnancy or are newly pregnant. These can be downloaded or ordered free-of-charge on the link below. We would like to thank Dr Onyebuchi Okosieme and Dr Anh Tran for their help with the development of this guidance.

Pregnancy alert card

More information on pregnancy and thyroid disease. Many patients also find our network of volunteer telephone contacts and local support groups a valuable resource.

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