What is the recommended treatment for first trimester hyperthyroidism?

First Trimester Hyperthyroidism Treatment

Propylthiouracil (PTU) is the preferred treatment for hyperthyroidism during the first trimester of pregnancy, with a goal of maintaining free T4 or FTI in the high-normal range using the lowest possible dosage. 1

Medication Selection and Rationale

• Thioamides (propylthiouracil or methimazole) are the standard treatment for hyperthyroidism in pregnancy 1
• Propylthiouracil is specifically preferred during the first trimester due to possible teratogenicity associated with methimazole in early pregnancy 1, 2
• The FDA notes that "propylthiouracil may be the treatment of choice when an antithyroid drug is indicated during or just prior to the first trimester of pregnancy" 2
• After the first trimester, switching to methimazole is recommended due to the risk of PTU-associated hepatotoxicity 1, 3

Dosing and Monitoring

• The goal of treatment is to maintain free T4 or free T4 index (FTI) in the high-normal range using the lowest possible thioamide dosage 1
• Measure free T4 or FTI every two to four weeks to guide dosage adjustments 1
• Until thioamide therapy reduces thyroid hormone levels, a beta blocker (e.g., propranolol) can be temporarily used to reduce symptoms 1
• TSH and FT4 or FTI testing should be performed for diagnosis and monitoring 1

Important Considerations and Risks

• Maternal risks of untreated hyperthyroidism: increased risk for severe preeclampsia, preterm delivery, heart failure, and possibly miscarriage 1
• Fetal risks of untreated hyperthyroidism: low birth weight 1
• Medication side effects to monitor:
  • Agranulocytosis (presents with sore throat and fever) - if suspected, obtain a complete blood count and discontinue the thioamide 1
  • Hepatitis, vasculitis, and thrombocytopenia are other potential side effects 1
  • Severe liver injury and acute liver failure have been reported with PTU, though this appears to be more common in the pediatric population 2

Special Situations

• Thyroidectomy should be reserved only for women who do not respond to thioamide therapy 1
• Radioactive iodine (I-131) is absolutely contraindicated in pregnancy 1, 4
• In women with Graves' disease, monitor fetal heart rate and growth; the newborn's physician should be informed about maternal Graves' disease due to the risk of neonatal thyroid dysfunction 1
• Hyperemesis gravidarum may be associated with biochemical hyperthyroidism (undetectable TSH level, elevated FTI), but this rarely requires treatment unless other signs of hyperthyroidism are present 1

Management Algorithm

1. Diagnosis: Confirm hyperthyroidism with TSH and free T4/FTI testing 1
2. First trimester treatment: Start propylthiouracil at the lowest effective dose 1, 2
3. Symptom control: Consider beta-blockers temporarily if symptoms are severe while waiting for thioamide effect 1
4. Monitoring: Check free T4 or FTI every 2-4 weeks to adjust dosage 1
5. Second trimester transition: Consider switching to methimazole after the first trimester to reduce hepatotoxicity risk 1, 3
6. Ongoing care: Monitor for medication side effects and maintain maternal euthyroidism 1

Pitfalls to Avoid

• Do not use radioactive iodine (I-131) under any circumstances during pregnancy 1, 4
• Do not delay treatment of overt hyperthyroidism as it poses significant risks to both mother and fetus 1, 3
• Do not overlook potential side effects of thioamides, particularly agranulocytosis and hepatotoxicity 1, 2
• Do not maintain unnecessarily high doses of thioamides, as suppression of fetal thyroid function can occur (though it is usually transient) 1