Management of Hyperthyroidism in Pregnancy on Methimazole
Immediate Action Required: Switch to Propylthiouracil
You should immediately switch this pregnant patient from methimazole to propylthiouracil (PTU) if she is still in the first trimester, as methimazole carries significant risk of congenital malformations including aplasia cutis, craniofacial defects, esophageal atresia, and omphalocele when used during this critical period. 1
Trimester-Specific Management Algorithm
First Trimester (Current Priority)
Discontinue methimazole immediately and switch to PTU, as methimazole crosses the placental membranes and causes rare but serious congenital defects including aplasia cutis, choanal atresia, esophageal atresia with or without tracheoesophageal fistula, and omphalocele 1
PTU is the preferred thioamide in the first trimester despite its higher risk of hepatotoxicity, because the teratogenic risk of methimazole outweighs PTU's liver toxicity risk during this critical developmental window 2, 1
Start PTU at an equivalent dose (methimazole 5mg is roughly equivalent to PTU 50mg), though exact conversion requires clinical judgment based on thyroid function 2
Second and Third Trimesters
Switch back to methimazole after the first trimester (around 16 weeks gestation), as PTU carries higher risk of severe hepatotoxicity including acute liver failure, particularly problematic during the longer exposure of later pregnancy 1, 3
Both propylthiouracil and methimazole can be used safely in the second and third trimesters, with recent studies showing no significant differences in mean FT4 or TSH levels in newborn cord-blood samples and similar rates of fetal anomalies 2
Target Thyroid Function During Pregnancy
Maintain FT4 or free thyroxine index (FTI) in the high-normal range using the lowest possible thioamide dosage to minimize fetal thyroid suppression while controlling maternal hyperthyroidism 2
With current values (T4 13.27, TSH 0.01), the patient appears inadequately controlled and may need dose adjustment of whichever thioamide is being used 2
Measure FT4 or FTI every 2-4 weeks during pregnancy to ensure adequate control without overtreatment 2
Critical Monitoring Requirements
Maternal Monitoring
Monitor for agranulocytosis by instructing the patient to immediately report fever or sore throat, as this is a potentially life-threatening adverse reaction requiring immediate drug discontinuation and bone marrow monitoring 1
Watch for symptoms of hepatic dysfunction including anorexia, pruritus, and right upper quadrant pain, prompting immediate evaluation of liver function (bilirubin, alkaline phosphatase, ALT, AST) 1
Discontinue the drug promptly if hepatic transaminase values exceed 3 times the upper limit of normal 1
Monitor maternal heart rate and ensure appropriate growth throughout pregnancy 2
Fetal Monitoring
Fetal thyroid suppression can occur with thioamide therapy but is usually transient and rarely requires treatment 2
Ultrasound screening for fetal goiter is not necessary unless problems with heart rate or growth are detected 2
The newborn's physician must be informed that the mother has Graves' disease because of the associated risk of neonatal thyroid dysfunction 2
Adjunctive Therapy Considerations
Beta blockers (e.g., propranolol) can be used to reduce symptoms of hyperthyroidism until thioamide therapy reduces thyroid hormone levels 2
This is particularly important given the current inadequate control (suppressed TSH, elevated T4) 2
Critical Pitfalls to Avoid
Never use radioactive iodine (I-131) during pregnancy, as it is absolutely contraindicated and can cause fetal thyroid ablation if exposure occurs after 10 weeks gestation 2
Do not aim for complete TSH normalization, as this may indicate overtreatment and increase risk of fetal hypothyroidism—the goal is high-normal FT4 with lowest possible drug dose 2
Avoid the common error of continuing methimazole throughout the first trimester when PTU should be used instead 1
Do not delay switching medications—the teratogenic window for methimazole-associated malformations occurs during organogenesis in the first trimester 1
Breastfeeding Considerations
Both propylthiouracil and methimazole are compatible with breastfeeding, so women treated with either agent can breastfeed safely 2
However, women should not breastfeed for 4 months after treatment with radioactive iodine if this was used prior to pregnancy 2
When Surgical Intervention Is Needed
- Thyroidectomy should be reserved for women who do not respond to thioamide therapy, ideally performed in the second trimester if surgery becomes necessary 2