Underactive Thyroid in Pregnancy: Guide
An underactive thyroid (hypothyroidism) means your thyroid gland is not making enough thyroid hormone. In pregnancy this matters because your baby relies on your thyroid hormones, especially in the early weeks, for healthy growth and brain development. The reassuring news is that hypothyroidism is common, easily monitored, and well managed with medication that is safe to take throughout pregnancy. This page explains the symptoms, treatment and monitoring you can expect.
Why your thyroid matters in pregnancy
In the first trimester your baby cannot yet make its own thyroid hormone and depends entirely on yours. Your body's demand for thyroid hormone also rises during pregnancy. If levels are too low and left untreated, hypothyroidism has been linked to problems such as miscarriage, pre-eclampsia and effects on your baby's development. Treating an underactive thyroid effectively brings these risks right down, which is why your team will want to keep your levels in a healthy range.
Symptoms of an underactive thyroid can include tiredness, feeling the cold, weight gain, dry skin, constipation and low mood. The tricky part is that many of these overlap with normal pregnancy feelings, so symptoms alone are not a reliable guide — blood tests are what show the true picture.
Treatment and monitoring
Hypothyroidism is treated with levothyroxine, a tablet that simply replaces the thyroid hormone your body is short of. It is safe to take in pregnancy, and it is important not to stop it. Take it on an empty stomach, usually first thing in the morning, and separate it from supplements such as iron and calcium, which can stop it being absorbed properly — leave a few hours between them.
If you already take levothyroxine, your dose often needs to increase during pregnancy because your body needs more. Many people are advised to increase their dose as soon as they know they are pregnant and then have it adjusted based on blood results, so contact your GP or midwife promptly for advice as soon as you get a positive test.
You will have regular blood tests (checking TSH and thyroid hormone levels) to fine-tune your dose, often around every four to six weeks in pregnancy or after any dose change, and more often if your levels are unstable. You may be looked after jointly by your maternity team and, in some cases, an endocrine (hormone) specialist. After birth, your dose is usually reviewed and often returns to your pre-pregnancy level.
What is normal and when to seek advice
It is normal to feel more tired in pregnancy even when your thyroid is well controlled, so try not to assume every symptom means your dose is wrong. Keep taking your medication, attend your blood tests, and raise any concerns at your appointments.
Contact your GP or midwife sooner rather than later if you think you might be pregnant and take levothyroxine, if you have a known thyroid condition and have just found out you are pregnant, or if you notice worsening symptoms such as extreme tiredness, palpitations, or significant changes in weight or mood. Seek urgent medical advice (call 111, or 999 if severe) for symptoms such as a very fast or irregular heartbeat, chest pain, or feeling seriously unwell, as your dose may need urgent review.
Frequently Asked Questions
How is an underactive thyroid treated in pregnancy?
With levothyroxine tablets, which replace the missing thyroid hormone and are safe in pregnancy. Your dose may need to increase during pregnancy, and regular blood tests (often every four to six weeks) are used to keep your levels in the right range.
Can an underactive thyroid harm my baby?
Untreated or poorly controlled hypothyroidism can affect your pregnancy and your baby's development. When it is properly treated and monitored, the risks are greatly reduced, which is exactly why staying on your medication and keeping up with blood tests matters.
Should I change my dose if I find out I'm pregnant?
Do not adjust it yourself, but contact your GP or midwife straight away. Many people need their levothyroxine dose increased early in pregnancy, and your team will guide the change based on your blood results.
For advice tailored to your own thyroid levels and pregnancy, please speak to your midwife or GP.
This article is for informational purposes only and does not constitute medical advice.
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