Thyroid Disease in Pregnancy

Why pregnancy changes the picture

The RCOG explains the basic physiology: the thyroid gland makes hormones "which control your metabolism and affect different organs including your heart, muscles and brain", and thyroid stimulating hormone (TSH) from the pituitary tells it how much to make. Then the crucial line: "When you are pregnant, your body usually makes more thyroxine for both you and your baby."

That single sentence is why a dose that worked perfectly before conception may not be enough now. It is also why "my thyroid is fine, I'm on treatment" is not the end of the conversation in early pregnancy.

The RCOG's headline reassurance is worth holding on to: "If your thyroid problem is well treated, you are highly likely to have an uncomplicated pregnancy and to give birth to a healthy baby."

Who gets tested

Thyroid testing in pregnancy is targeted, not universal. The RCOG says you will be offered a blood test in the first trimester to check your thyroid function if:

• you have had thyroid problems in the past
• you have had an operation on your thyroid gland
• you have a swelling in your thyroid gland
• you have another autoimmune disease such as type 1 diabetes, lupus or antiphospholipid syndrome

It adds that you will also be offered a thyroid blood test if you have had a stillbirth or late miscarriage and have not been tested before.

If you fall into one of those groups and no test has been arranged, ask. It is a single blood test and the window in which it is most useful is early.

Underactive thyroid (hypothyroidism)

The RCOG describes hypothyroidism as the body not making enough thyroxine, and lists the symptoms: tiredness, increased awareness of cold, dry skin and hair, constipation, and heavy or irregular periods or fertility problems. The NHS's page on underactive thyroid covers the same ground outside pregnancy — and notes how easily these symptoms are attributed to something else.

On risk, the RCOG is direct in both directions. "If your hypothyroidism is well treated it will not cause any harm to you or your baby." But untreated hypothyroidism in pregnancy "may be linked with a higher chance of" miscarriage, premature birth, a low birthweight baby, and effects on the baby's brain development.

Treatment is levothyroxine, which the RCOG says is safe in pregnancy. The two practical points are about dose and monitoring: "If you are already taking Levothyroxine tablets before becoming pregnant, your healthcare professional will discuss changing your dose when you become pregnant", and you will be offered thyroid blood tests at your first midwife appointment and regularly through pregnancy to check you are on the right dose.

Subclinical hypothyroidism — a normal thyroxine level with a high TSH — is a judgement call. The RCOG says your clinician will discuss whether it needs treatment, and that severe subclinical hypothyroidism diagnosed in pregnancy is treated with levothyroxine as soon as possible.

After birth, the RCOG says you can usually go back to your pre-pregnancy dose, that people started on levothyroxine during pregnancy may be able to stop, and that thyroid blood tests should be checked again when your baby is 6 weeks old.

Overactive thyroid (hyperthyroidism)

The RCOG lists the symptoms: weight loss, heart palpitations, intolerance to heat, shakiness, mood swings, an enlarged thyroid gland and very prominent eyes. The NHS's overactive thyroid page covers the general condition.

The most common cause is Graves' disease, which the RCOG describes as "an autoimmune condition where there are thyroid antibodies in your blood that cause the thyroid gland to make more thyroxine than it needs to."

Poorly controlled hyperthyroidism in pregnancy is linked by the RCOG to a higher risk of pre-eclampsia, premature birth, low birthweight and stillbirth, and it notes a higher chance of the baby having additional learning needs, autism or ADHD.

Treatment in pregnancy is usually anti-thyroid tablets — propylthiouracil or carbimazole. The RCOG notes that "some of these tablets are safer for your baby's development than others" and that the lowest possible dose will be advised. Which drug is used and when it is switched is a specialist decision, and it is one of the main reasons to plan a pregnancy with your endocrine team rather than after the fact.

If you become pregnant unexpectedly on anti-thyroid tablets, the RCOG says to tell your healthcare professional straight away so a personalised plan can be made. Do not stop the tablets yourself.

Graves' antibodies and the baby

This is the part of thyroid care in pregnancy that is genuinely different, and it applies even if your own thyroid has been treated or removed.

The RCOG: "If you have Grave's disease and have high thyroid antibody levels there is a chance of your baby getting hyperthyroidism before or soon after they are born. In this situation, your baby will be monitored more closely before and after birth."

That means antibody levels are measured as well as thyroid function, and it may mean extra growth scans. The RCOG says regular blood tests will check both your thyroid function and your antibody levels, and that the type and dose of medication may change based on them.

Iodine

The RCOG covers iodine because deficiency is a cause of thyroid problems and because pregnancy raises requirements. It gives the recommended daily amount when planning a pregnancy, during pregnancy and while breastfeeding as 200 to 250 micrograms, and lists cow's milk, yoghurt, eggs and fish such as cod or haddock as sources. It notes that common pregnancy multivitamins contain 150 micrograms.

The warning is as important as the recommendation: "It is important not to take too much iodine as this can be harmful to you and your baby, so you should avoid taking multiple different vitamin supplements." If you follow a vegan diet, the RCOG suggests asking your clinician whether an iodine supplement would help — rather than stacking supplements yourself.

Birth and afterwards

The RCOG says well-controlled hypothyroidism "will not change how or when you give birth", and the same applies to well-controlled hyperthyroidism. Where Graves' disease is active or antibody levels are high, you may be advised to give birth on a labour ward with continuous monitoring of the baby's heartbeat.

Detailed UK clinical practice follows the RCOG's Green-top Guideline No. 76 on the management of thyroid disorders in pregnancy. ACOG's practice bulletin on thyroid disease in pregnancy sets out US practice, and the British Thyroid Foundation has patient-facing information on pregnancy and fertility in thyroid disorders, including what to expect from monitoring.

One practical thing to do now: if you take levothyroxine and you have just found out you are pregnant, contact whoever prescribes it this week. A dose review early is worth far more than a perfect one at 20 weeks.