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Date of issue: February 2024, Version: 3.0

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A corresponding patient information leaflet on USE OF IODINE 131 IN PREGNANCY is available.

Iodine 131 (I-131) is a radioactive iodine nuclide that emits beta particles and a small amount of gamma particles. It is widely used in nuclear medicine imaging studies of the thyroid gland and in the management of hyperthyroidism and thyroid cancer.

I-131 therapy potentially exposes the fetus to ionising radiation and is therefore contraindicated in pregnancy. Given that the fetal thyroid begins functioning at approximately 8-10 weeks post-conception (gestational weeks 10 to 12), a delay between I-131 treatment and pregnancy is usually suggested to avoid radioactive iodine accumulation in the fetal thyroid. UK guidelines currently recommend that women avoid pregnancy for at least six months following treatment.

The available human pregnancy exposure data consist mainly of uncontrolled studies which describe pregnancy outcomes following pre-conception exposure, with only a small number of uncontrolled studies and case reports describing the outcomes of pregnancies where I-131 exposure occurred during pregnancy.

There is currently no conclusive evidence that exposure to I-131 either prior to conception or during pregnancy is associated with the occurrence of fetal congenital malformation, miscarriage, impaired fetal growth, preterm delivery, or stillbirth. However, the available data are limited and of relatively poor quality; as such, it is recommended that pregnancy is avoided after I-131 treatment in accordance with current guidelines.

There have been a number of case reports of congenital hypothyroidism following I-131 treatment of maternal thyrotoxicosis or thyroid cancer. The absolute risk of congenital hypothyroidism in the offspring following I-131 exposure during pregnancy remains unquantified. Neurodevelopmental impairment with congenital hypothyroidism has also been reported in a small number of infants exposed to I-131 in utero. However, this may have been influenced by delays in the diagnosis of congenital/neonatal hypothyroidism and/or in the administration of neonatal thyroxine supplementation. Monitoring of the neonatal thyroid function is advised for all cases of in utero I-131 exposure.

Rates of thyroid carcinoma were not significantly increased amongst the offspring of women exposed to I-131 during the Chernobyl disaster when compared to an unexposed cohort, and no dose-response relationship between in utero I-131 fallout exposure in pregnancy and thyroid carcinoma in the offspring has been identified. No cases of thyroid cancer have been reported in offspring of women treated with I-131 prior to conception or during pregnancy.

Monitoring of neonatal thyroid function is advisable where inadvertent exposure to I-131 therapy has occurred during pregnancy. Other risk factors may also be present in individual cases which independently increase the risk of adverse pregnancy outcome. Clinicians are reminded of the importance of consideration of such factors when performing case-specific risk assessments.

This is a summary of the full UKTIS monograph for health care professionals and should not be used in isolation. The full UKTIS monograph and access to any hyperlinked related documents is available to NHS health care professionals who are logged in.

If you have a patient with exposure to a drug or chemical and require assistance in making a patient-specific risk assessment, please telephone UKTIS on 0344 892 0909 to discuss the case with a teratology specialist.

If you would like to report a pregnancy to UKTIS please click here to download our pregnancy reporting form. Please encourage all women to complete an online reporting form.

Disclaimer: Every effort has been made to ensure that this monograph was accurate and up-to-date at the time of writing, however it cannot cover every eventuality and the information providers cannot be held responsible for any adverse outcomes of the measures recommended. The final decision regarding which treatment is used for an individual patient remains the clinical responsibility of the prescriber. This material may be freely reproduced for education and not for profit purposes within the UK National Health Service, however no linking to this website or reproduction by or for commercial organisations is permitted without the express written permission of this service. This document is regularly reviewed and updated. Only use UKTIS monographs downloaded directly from to ensure you are using the most up-to-date version.