Thyroid conditions in pregnancy: what to know and what is monitored

The thyroid is a small gland in the neck that produces hormones controlling the body’s metabolism. The British Thyroid Foundation (BTF) explains that an undiagnosed or uncontrolled thyroid disorder can make it harder to conceive and can cause problems during pregnancy. For women, it says, a normally functioning thyroid is essential for ovulation, implantation and maintenance of a healthy pregnancy.

The most common thyroid disorder in women of childbearing age, according to the BTF, is hypothyroidism, an underactive thyroid. The foundation notes that low levels of thyroid hormone can interfere with the release of an egg from the ovaries, and that some underlying causes, such as Hashimoto’s thyroiditis, also appear to impair fertility. Overactive thyroid (hyperthyroidism) can also make conception harder and, if untreated, increases the risk of miscarriage and of premature delivery, according to the foundation’s frequently asked questions.

The NHS advises anyone with an underactive thyroid who is pregnant or planning to get pregnant to tell their GP. It says you will need further tests and it is likely your dose of levothyroxine will need to change. The BTF suggests speaking to your GP to arrange thyroid blood tests, with a goal of a thyroid-stimulating hormone (TSH) level of less than 2.5 mU/l at the time of conception and in the first trimester, and less than 3.0 mU/l afterwards. Targets can differ between clinics, so follow the advice of the clinician looking after you.

If you have an overactive thyroid, the foundation says to discuss your plans with your endocrinologist before you try to conceive, as medication may need to change or in some cases be stopped. If you have been treated with radioiodine, its FAQ says not to become pregnant for six months afterwards, and that men should avoid fathering a child for at least four months.

The foundation also advises that women who have a history of thyroid disease, even if not on treatment now, should speak to their GP and arrange thyroid blood tests as soon as they have a positive pregnancy test. The history it lists includes subclinical hypothyroidism, thyroiditis, radioactive iodine treatment, thyroidectomy, goitre and positive thyroid antibodies.

The BTF says that, as soon as you know you are pregnant, it is generally recommended that your levothyroxine is increased immediately, usually by 25 to 50 micrograms a day, which can be achieved by doubling your dose on two days of the week. You should then contact your GP and arrange a thyroid blood test. The exception it names is women treated for thyroid cancer whose doses are intended to suppress TSH; they may not need an increase but should discuss it with their GP or specialist. Do not change your dose without being told it is appropriate for you.

Monitoring is repeated: the foundation advises tests every four to six weeks during pregnancy and again a few weeks after delivery, though less frequent testing may be reasonable in later pregnancy if levels have been stable. After the birth most women return to their pre-pregnancy dose, and those taking levothyroxine for subclinical hypothyroidism may be able to stop, but that should be agreed with a GP or specialist before changing anything.

The foundation adds that breastfeeding is safe while taking levothyroxine, and that you can take antenatal vitamins but should avoid taking them at the same time as your thyroid tablet because some contain calcium and iron that interfere with absorption.

The BTF explains that there is a small risk of birth defects with all antithyroid drugs in pregnancy, but taking them is safer for mother and baby than uncontrolled thyroid overactivity. Because the drugs cross the placenta, the lowest possible dose is used. In its guidance, propylthiouracil is described as the drug of choice when trying to conceive and in the first three months, with carbimazole used for the rest of pregnancy. Your endocrinologist will decide what is right for you.

Blood tests may be needed every four weeks throughout pregnancy, though not everyone needs checks so often. The foundation says anyone taking antithyroid drugs who develops a rash, sore throat or unexplained fever must see a doctor immediately and ask for a full blood count, because of a small risk of agranulocytosis, and that itching or yellowing of the eyes with propylthiouracil should also prompt urgent medical attention.

For women with Graves’ disease, the foundation’s FAQ explains that TSH receptor antibodies (TRAb) can cross the placenta and affect the baby, so if they are positive, referral to an antenatal clinic for monitoring is recommended. It notes that on rare occasions this causes temporary overactivity in the baby that is treatable.

The BTF says postpartum thyroiditis is usually a temporary disorder that occurs in women with positive thyroid antibodies, typically showing up in the first six months after birth. It often starts with symptoms of an overactive thyroid and may resolve by itself or progress to an underactive phase. Mild cases may need no treatment, a beta blocker may relieve overactive symptoms, and levothyroxine may be prescribed for underactive symptoms. The FAQ states that in around one in three cases the thyroid disease remains permanent.

Because the condition can recur after later pregnancies, the foundation advises having thyroid blood tests two to three months after each birth and a yearly check afterwards where it has occurred. Overactive thyroid can also return in the first year after birth, so tests around three months after delivery are advised for women with a history of hyperthyroidism.

Congenital hypothyroidism in newborns is rare, about one in 2000 to 3000 babies in the UK according to the foundation. Your midwife or GP can explain what applies to your baby.

In the NHS, thyroid care in pregnancy is usually shared between your GP, midwife and, for more complex or overactive thyroid disease, an endocrinologist. Your booking appointment is a good time to mention any thyroid history. The BTF also provides a pregnancy and thyroid disorders alert card that can be shown to staff to help you get appropriate monitoring.

Some women use private endocrinology or antenatal services for faster access to testing or specialist advice. The medical principles are the same, and your NHS team should be told about any changes to medicines or results. Contact your GP or midwife promptly if you are unsure whether to adjust your tablets, if you have new symptoms such as a racing heart, marked tiredness or a swelling in the neck, or if you are unwell and unable to keep your tablets down. For emergencies, use 999 or NHS 111 as appropriate.