USE OF IODINE 131 IN PREGNANCY

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 to 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 is limited and of relatively poor quality; as such, it is recommended that pregnancy is avoided after I-131 treatment.

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. 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.