SAT-410 - A Case of Radioactive Iodine Exposure during Early Pregnancy for Management of Toxic Adenoma

Background: Toxic adenomas are the least common causes of hyperthyroidism. Patients are typically asymptomatic and their biochemical profile is consistent with sub-clinical hyperthyroidism. Once the toxic adenoma reaches a diameter of 3 cm or more, patients become overtly hyperthyroid. There are multiple treatment options for the management of toxic adenomas, including radioactive iodine (RAI) ablation, surgical excision, and thermal ablation. RAI therapy uses ionizing radiation to destroy follicular cells with 50% of the patients becoming hypothyroid within one year post treatment. Despite its effectiveness, RAI must be used with caution in women of childbearing potential. Its use is contraindicated in pregnancy, as fetuses exposed to I-131 after 10 weeks of gestation may develop congenital hypothyroidism. Due to this safety concern, women should wait at least 6 months following treatment with RAI before conceiving to allow RAI to decay and the thyroid function to stabilize.

Clinical

Case: A 23-year-old female was referred to endocrinology for subclinical hyperthyroidism that was diagnosed two years prior. She started on methimazole 5 mg daily but had poor medication compliance. She described symptoms of unintentional weight loss associated with palpitations. Biochemical testing showed a TSH of 0.29 [0.45-4.5 uIU/mL] with FT4 of 1.3 [0.60-1.30 ng/dL]. Thyroid ultrasound showed a solid, hypoechoic, left mid-pole thyroid nodule 1.5 X 1.1 X 1.2 cm. A thyroid uptake scan showed mildly elevated radioactive iodine thyroid uptake with a focus of increased radiotracer in the left thyroid lobe, consistent with a hyperfunctioning nodule. She was referred to RAI ablation, receiving 25 mCi of I-131. Twelve weeks post- RAI she had a positive pregnancy test. Biochemical testing at the time revealed a TSH of 114.00 [0.45-4.5 uIU/mL] with a low FT4 of 0.10 [0.60-1.30 ng/dL]. Due to the severity of her hypothyroidism in the setting of pregnancy, she was treated with hydrocortisone 100 mg, followed by a loading dose of IV levothyroxine. She was eventually transitioned to oral levothyroxine 112 mcg daily and she decided to carry her pregnancy to term. Patient later reported she had an uncomplicated pregnancy and delivered a healthy baby boy who, at 9 months of age, reached all his developmental milestones.


Conclusion: RAI is a safe and effective treatment modality for toxic adenomas. However, inadvertent exposure to I-131 in utero particularly during organogenesis can potentially increase the risk for congenital hypothyroidism which can lead to neurodevelopmental disorders including intellectual disability. Because of this potential risk, women of childbearing age should be informed about them and use an effective form of contraception to avoid potential complications in the event of pregnancy.

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