Iodine 131

Date: March 2024, Version 3

Quick read

Iodine 131 is generally avoided in pregnancy as it can damage the baby’s thyroid gland and may affect the baby’s development.

What is iodine 131?

Iodine 131 is a radioactive form of iodine. Iodine in the body is naturally stored in the thyroid gland. Small doses of iodine 131 are used to picture the thyroid in medical scans. Iodine 131 is also used at higher doses to destroy excess thyroid tissue in the treatment of hyperthyroidism (overactive thyroid) and thyroid cancer.


What are the benefits of using iodine 131 in pregnancy?

Use of iodine 131 in pregnancy is not recommended.


What are the risks of using iodine 131 in pregnancy?

Radiation from iodine 131 given in the first trimester could cause birth defects or a miscarriage. From 10-12 weeks of pregnancy, iodine 131 can damage the baby’s developing thyroid gland.

As iodine 131 can stay in the body for some time, it is recommended that women avoid getting pregnant for six months after exposure.  

As a precaution, it is also recommended that pregnant women avoid prolonged contact with other people who have received iodine 131 within the past three weeks (such as sitting or sleeping next to them for several hours a day) to ensure that the baby is not exposed to too much radiation. Shorter-term contact with people who have received iodine 131 is not known to be unsafe for the baby.


Are there any alternatives to receiving iodine 131 during pregnancy?

Yes. If you are pregnant and need a thyroid scan or treatment for a thyroid condition, your doctor should be able to offer alternatives to iodine 131.

Will my baby need extra monitoring during pregnancy?

As part of their routine antenatal care, most women will be offered a scan at around 20 weeks of pregnancy to look for birth defects and to check the baby’s growth. 

If you were exposed to iodine 131 during pregnancy, or you conceived within 6 months of treatment, you should tell your doctor or specialist as soon as possible. You may need additional scans to check the baby’s growth, development, and thyroid gland. Women with a history of thyroid cancer or other thyroid problems will be offered additional monitoring during pregnancy to ensure that their thyroid hormone levels stay at correct levels to support their own and their baby’s wellbeing.

Are there any risks to my baby if the father was treated with iodine 131?

Men who are treated with iodine 131 may wish to avoid conception for six months after treatment due to possible risks of radiation exposure on sperm development. However, small studies have not shown any ill effects on pregnancies fathered by men who received iodine 131 prior to conception. For more information on medicine exposure in fathers around the time of conception please see the bump leaflet on Paternal exposures.

Who can I talk to if I have questions?

If you have any questions regarding the information in this leaflet, please discuss them with your health care provider. They can access more detailed medical and scientific information from

How can I help to improve drug safety information for pregnant women in the future?

Our online reporting system (MyBump Portal) allows women who are currently pregnant to create a secure record of their pregnancy, collected through a series of questionnaires. You will be asked to enter information about your health, whether or not you take any medicines, your pregnancy outcome and your child's development. You can update your details at any time during pregnancy or afterwards. This information will help us better understand how medicines affect the health of pregnant women and their babies. Please visit the MyBump Portal to register.

General information
Sadly, miscarriage and birth defects can occur in any pregnancy.

Miscarriage occurs in about 1 in every 5 pregnancies, and 1 in every 40 babies are born with a birth defect. This is called the ‘background risk’ and happens whether medication is taken or not.

Most medicines cross the placenta and reach the baby. For many medications this is not a problem. However, some medicines can affect a baby’s growth and development.

If you take regular medication and are planning to conceive, you should discuss whether your medicine is safe to continue with your doctor/health care team before becoming pregnant. If you have an unplanned pregnancy while taking a medicine, you should tell your doctor as soon as possible.

If a new medicine is suggested for you during pregnancy, please make sure that the person prescribing it knows that you are pregnant. If you have any concerns about a medicine, you can check with your doctor, midwife or pharmacist.

Our Bumps information leaflets provide information about the effects of medicines in pregnancy so that you can decide, together with your healthcare provider, what is best for you and your baby.