Iodine 131

(Date: January 2019. Version: 2)

This factsheet has been written for members of the public by the UK Teratology Information Service (UKTIS). UKTIS is a not-for-profit organisation funded by the UK Health Security Agency (UKHSA) on behalf of UK Health Departments. UKTIS has been providing scientific information to health care providers since 1983 on the effects that medicines, recreational drugs and chemicals may have on the developing baby during pregnancy.

What is it?

Iodine 131 is a radioactive form of iodine. Because iodine accumulates in the thyroid gland, iodine 131 is given at low doses during some types of thyroid scan to allow visualisation of the thyroid. It is also used at higher doses to destroy thyroid cells in the treatment of thyroid cancer, and some types of hyperthyroidism (overactive thyroid).

Is exposure to iodine 131 safe in pregnancy?

Exposure to iodine 131 in pregnancy is not recommended. If exposure occurs in the first trimester, the radiation emitted by iodine 131 can theoretically harm the developing baby and may cause birth defects or a miscarriage. From 10-12 weeks of pregnancy, iodine 131 can accumulate in and potentially damage the baby’s developing thyroid gland. For this reason, UK guidelines suggest that women receiving iodine 131 should avoid conception for six months after treatment. 

This leaflet summarises the available scientific information on women who received treatment with iodine 131 in pregnancy.

What if I have already received iodine 131 during pregnancy?

If you were exposed to iodine 131 without realising you are pregnant, 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 that the baby is growing and developing as expected, and that the baby’s thyroid gland is normal. If you are taking any other medicine it is also a good idea to let your doctor know that you are pregnant so that you can decide together whether you still need the medicines that you are on and, if so, to make sure that you are taking the lowest dose that works and only for as long as you need to.

What if a member of my household or a close contact has received iodine 131 treatment?

As a precaution, it is recommended that pregnant women avoid prolonged contact with other people who have received iodine 131 within the previous three weeks (such as sitting or sleeping next to a person for several hours a day), as they will theoretically emit radiation at a level that could be harmful. Shorter-term contact with people who have received iodine 131 is not thought to pose a risk.

Can exposure to iodine 131 in pregnancy cause my baby to be born with birth defects?

A baby’s body and most internal organs are formed during the first 12 weeks of pregnancy. It is during this time that some medicines can cause birth defects.

There is a theoretical risk that the radiation from iodine 131 could cause birth defects during the first trimester of pregnancy. A small number of small studies have assessed the chance of birth defects in babies born to women who were exposed to iodine 131 either just before or during early pregnancy, with none finding a link with birth defects in the baby. While this is reassuring, more research is ideally required to confirm this finding.

Can exposure to iodine 131 in pregnancy cause miscarriage?

There is a theoretical risk that the radiation emitted by iodine 131 could cause miscarriage. No studies have assessed the chance of miscarriage in women receiving iodine 131 during pregnancy. A number of case reports do not raise concern that miscarriage is more common following exposure to iodine 131 in early pregnancy, but more research is needed.

Can exposure to iodine 131 in pregnancy cause stillbirth?

No studies have investigated the chance of stillbirth following exposure to iodine 131 before or during pregnancy. While a small number of case reports do not raise concerns of any links, this needs to be confirmed with further research.

Can exposure to iodine 131 in pregnancy cause preterm birth?

No studies have assessed the chance of preterm birth following treatment with iodine 131 during pregnancy. Three of four studies of women who conceived soon after treatment with iodine 131 found no link with preterm birth.

Can exposure to iodine 131 in pregnancy cause other problems in the baby/child?

Thyroid problems
Exposure in the womb to iodine 131 starting from 10-12 weeks of pregnancy can damage the baby’s developing thyroid gland. No studies have investigated the precise chance of this happening, but there are a number of case reports of exposed babies born with thyroid problems, specifically an underdeveloped/underactive thyroid (‘congenital hypothyroidism’).

Learning or behavioural problems
A baby’s brain continues to develop right up until the end of pregnancy. It is therefore possible that taking certain medicines at any stage of pregnancy could have a lasting effect on a child’s learning or behaviour.

No studies have investigated the chance of learning and behavioural problems in children exposed to iodine 131 in the womb. In theory, congenital hypothyroidism, due to iodine 131 exposure in the womb, could affect the baby’s brain development both before and after birth, leading to lower intelligence.

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.

Women who have been exposed to iodine 131 during pregnancy, or within the six months before conception, may be offered additional scans to ensure the baby is growing and developing as expected and to check the baby’s thyroid. 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 within recommended limits to support both their 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 hypothetical risks about the effects of radiation exposure on sperm development. However, small studies have not shown any adverse 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. As discussed above, if a close contact receives iodine 131 treatment during your pregnancy, prolonged contact with the person should be avoided for three weeks after treatment as a precautionary measure.

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

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General information 

Up to 1 out of every 5 pregnancies ends in a miscarriage, and 1 in 40 babies are born with a birth defect. These are referred to as the background population risks.  They describe the chance of these events happening for any pregnancy before taking factors such as the mother’s health during pregnancy, her lifestyle, medicines she takes and the genetic make up of her and the baby’s father into account.

Medicines use in pregnancy

Most medicines used by the mother will cross the placenta and reach the baby. Sometimes this may have beneficial effects for the baby.  There are, however, some medicines that can harm a baby’s normal development.  How a medicine affects a baby may depend on the stage of pregnancy when the medicine is taken. If you are on regular medication you should discuss these effects with your doctor/health care team before becoming pregnant.

If a new medicine is suggested for you during pregnancy, please ensure the doctor or health care professional treating you is aware of your pregnancy.

When deciding whether or not to use a medicine in pregnancy you need to weigh up how the medicine might improve your and/or your unborn baby’s health against any possible problems that the drug may cause. Our bumps leaflets are written to provide you with a summary of what is known about use of a specific medicine in pregnancy so that you can decide together with your health care provider what is best for you and your baby.   

Every pregnancy is unique. The decision to start, stop, continue or change a prescribed medicine before or during pregnancy should be made in consultation with your health care provider. It is very helpful if you can record all your medication taken in pregnancy in your hand held maternity records.

Disclaimer: This information is not intended to replace the individual care and advice of your health care provider. New information is continually becoming available. Whilst every effort will be made to ensure that this information is accurate and up to date at the time of publication, we cannot cover every eventuality and the information providers cannot be held responsible for any adverse outcomes following decisions made on the basis of this information. We strongly advise that printouts should NOT be kept for any length of time, or for “future reference” as they can rapidly become out of date.

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