Pregnancy testing prior to iodine-131 thyroid therapy: a literature review of current recommendations

Iodine-131 (I-131) thyroid ablation is strictly contraindicated during pregnancy due to the risk of fetal thyroid damage after 10 weeks of gestation. Current society guidelines lack consistency regarding the best timing and method for pregnancy testing. This review aims to consolidate current recommendations and propose a standardized testing algorithm.

A literature review was conducted using the PubMed/MEDLINE database over a 55-year period to identify peer-reviewed publications on professional society guidelines for pre-radioiodine therapy pregnancy testing, as well as performance characteristics of serum and urine pregnancy tests.

Fetal susceptibility to radioiodine begins around 8-10 weeks of gestation. While serum hCG tests are approximately 2.5 times more sensitive than standard urine tests, they still have a "biological window" during which early pregnancy might be undetectable. Current guidelines differ on testing timelines, ranging from 24 to 72 h before therapy.

Patients should be advised to abstain from sexual activity for at least two weeks before therapy and provide a written attestation on the day of treatment confirming they completed this abstinence period. Serum pregnancy testing is preferred because of its higher sensitivity and should be performed within 72 h prior to therapy, ideally within 24 h.