Pregnancy, breastfeeding and fertility while taking carbimazole

Carbimazole and pregnancy

It's important to correctly treat a thyroid problem in pregnancy. If your thyroid hormone levels are too high or too low, this can cause problems for your baby.

Carbimazole is not often recommended during pregnancy. You'll usually be switched to another thyroid medicine called propylthiouracil (PTU).

This is because carbimazole can very occasionally cause problems for your baby's development in early pregnancy. However, not everyone can take PTU and in some cases, continuing carbimazole may be the safest option.

If you become pregnant while taking carbimazole, keep taking it and speak to your doctor as soon as possible. You will be invited to see a pregnancy specialist (obstetrician) and a hormone specialist (endocrinologist). They can advise you on the best treatment options for you and your baby.

If you continue taking carbimazole in pregnancy, you'll need regular blood tests to check you're on the right dose. You may also have extra scans to check the health of your growing baby.

Carbimazole and breastfeeding

If your baby is healthy, it's usually OK to take carbimazole while you are breastfeeding, and it's important to treat your thyroid problem.

Carbimazole gets into breast milk in small amounts. It's unlikely to cause any side effects in your baby or affect their thyroid hormones.

It's very rare for breastfed babies to have side effects from medicines you're taking. But it's important to keep monitoring your baby if you're taking any medicine while breastfeeding.

Contact your healthcare professional or call 111 as soon as possible if you notice that your baby:

• is not feeding as well as usual or not putting on weight as you would expect
• is unusually sleepy or irritable
• has a rash
• has difficulty pooing (constipation)
• gets infections regularly

Your doctor may take a blood test from your baby to check everything is OK.

If your baby has any symptoms that are not listed, or if you're worried about them, you can also call 111 or contact any healthcare professional.

Carry on taking the medicine, and keep breastfeeding as usual until you've got advice about your baby's symptoms.

Carbimazole and fertility

There's no evidence to suggest that taking carbimazole will reduce fertility in either men or women.

However, speak to your doctor if you're trying to get pregnant as it's important not to get pregnant while you're taking carbimazole. Use contraception until your medicine has been reviewed.