SAT-330 - Successful Pregnancy Outcome Following Unintentional First-Trimester Methimazole Exposure in Graves' Disease: A Case Report

Background: Methimazole exposure during the first trimester of pregnancy is associated with an increased risk of congenital malformations, including aplasia cutis, esophageal atresia, and abdominal wall defects. Current guidelines recommend propylthiouracil as the preferred antithyroid drug during early pregnancy. However, unintentional methimazole exposure occurs when women become pregnant while on established therapy. We present a case demonstrating successful pregnancy outcome despite first-trimester methimazole exposure.

Case Presentation:
A 20-year-old female with Graves’ hyperthyroidism was well controlled on methimazole 10 mg daily. She discovered her unplanned pregnancy at approximately four weeks gestational age and discontinued MMI immediately. First-trimester laboratory evaluation showed TSH 1.189 mIU/L, free T4 0.86 ng/dL, total T3 93 ng/dL, TSI 7.57 IU/L, and TRAb 4.98 IU/L. After shared decision-making and discussion with OB, the patient elected to remain off antithyroid therapy with monthly monitoring.
At 26 weeks’ gestation, labs revealed TSH 1.298 mIU/L, free T4 0.79 ng/dL, total T3 151 ng/dL, TSI 3.03 IU/L, and TRAb 1.42 IU/L. Fetal growth was appropriate with heart rates in the 140s–150s and no evidence of fetal hyperthyroidism. At 31 weeks’ gestation, thyroid function remained stable (TSH 0.974 mIU/L, free T4 0.83 ng/dL, total T3 148 ng/dL), with declining TSI (1.92 IU/L) and negative TRAb ( < 1.10 IU/L). Fetal assessment remained reassuring. She remained off of antithyroid drugs for the remainder of her pregnancy as she was clinically euthyroid.
The patient experienced a postpartum flare of Graves’ disease and was restarted on methimazole 10 mg daily. She breastfed without thyroid-related complications in the infant or evidence of fetal hyperthyroidism.
Discussion
This case highlights one approach to patients who have unintentional pregnancies on methimazole. Studies have shown that there is an increased relative risk (1.28) of birth defects for neonates exposed to methimazole and propylthiouracil (1.16) in utero compared to unexposed controls. Therefore clinicians try to counsel patients to have definitive treatment to achieve euthyroidism and Graves’ remission prior to pregnancy, but if this is not possible, to switch to propylthiouracil or wean off methimazole prior to pregnancy. However, this case shows that if a patient has an unintended pregnancy on methimazole, providers can counsel patients on stopping methimazole at that point, and monitoring both the fetus and the mother with non-invasive thyroid testing and fetal monitoring for the remainder of the pregnancy.
In addition, available data has shown that thyroid function is normal in infants exposed to lower doses of methimazole via breast milk. Our case study supports the evidence that breast feeding while on methimazole is safe at lower doses under 20mg.

*Unless otherwise noted, all abstracts presented at ENDO must not be released to the press or the public until the date and time of presentation. For oral presentations, the abstracts are embargoed until the session begins. The Endocrine Society reserves the right to lift the embargo on specific abstracts that are selected for promotion prior to or during ENDO.*