Treatment of Hyperthyroidism in Pregnancy
Propylthiouracil (PTU) is the first-line antithyroid medication during the first trimester of pregnancy, with a switch to methimazole recommended for the second and third trimesters to minimize both fetal teratogenic risk and maternal hepatotoxicity. 1, 2
Medication Selection by Trimester
First Trimester
- Use PTU exclusively during the first trimester due to lower risk of congenital abnormalities compared to methimazole 1, 2
- Methimazole causes a specific pattern of rare teratogenic effects including choanal atresia and aplasia cutis congenita when used during organogenesis 3
- The FDA has issued a black box warning for PTU regarding severe liver injury, but notes it "may be the treatment of choice when an antithyroid drug is indicated during or just prior to the first trimester of pregnancy" 4
Second and Third Trimesters
- Switch from PTU to methimazole after the first trimester to reduce maternal hepatotoxicity risk 1, 5
- PTU carries risk of severe, potentially fatal hepatic failure requiring liver transplantation 4, 3
- Both drugs cross the placenta equally, so the switch is based on safety profile rather than placental transfer differences 6
Treatment Goals and Monitoring
Target Thyroid Function
- Maintain free T4 or free thyroxine index (FTI) in the high-normal range using the lowest possible thioamide dosage 1, 2
- Monitor free T4 or FTI every 2-4 weeks to adjust medication dosage 1, 2
- Check TSH level every trimester once stable thyroid function is achieved 1, 2
Symptomatic Management
- Beta-blockers (propranolol) can be used temporarily to control tremors and palpitations until thioamide therapy reduces thyroid hormone levels 1
- As patients become euthyroid, beta-blocker doses may need reduction due to decreased clearance 7
Critical Safety Monitoring
Maternal Monitoring
- Watch for signs of agranulocytosis: sore throat and fever 1
- Monitor for hepatotoxicity, particularly with PTU use 4
- Monitor prothrombin time, especially before surgical procedures, as methimazole may cause hypoprothrombinemia 7
Fetal Monitoring
- Monitor for normal fetal heart rate and appropriate growth in women with Graves' disease 2
- Inform the newborn's physician about maternal Graves' disease due to risk of neonatal thyroid dysfunction 1, 2
Surgical Intervention
Indications for Thyroidectomy
- Reserve thyroidectomy for women who do not respond to thioamide therapy 1, 2
- Consider surgery for patients with severe drug intolerance (agranulocytosis or severe hepatotoxicity) 1
- Perform surgery during the second trimester if necessary 1
Absolute Contraindications
- Radioactive iodine (I-131) is absolutely contraindicated during pregnancy as it causes fetal thyroid ablation 1, 2
- Women must wait four months after I-131 treatment before breastfeeding 8, 2
Special Clinical Scenarios
Hyperemesis Gravidarum
- Biochemical hyperthyroidism associated with hyperemesis gravidarum rarely requires treatment unless other clinical signs of hyperthyroidism are present 8, 2
- Routine thyroid testing is not recommended unless other hyperthyroid signs exist 8
Thyroid Storm
- This medical emergency presents with fever, disproportionate tachycardia, altered mental status, vomiting, diarrhea, and cardiac arrhythmia 8
- Do not delay treatment for laboratory confirmation 8
- Treatment includes: PTU or methimazole, potassium/sodium iodide solutions, dexamethasone, phenobarbital, and supportive care 8, 2
- Avoid delivery during thyroid storm unless absolutely necessary 8
Consequences of Inadequate Treatment
- Untreated or inadequately treated hyperthyroidism increases risk of preeclampsia, preterm delivery, heart failure, miscarriage, stillbirth, and low birth weight 1, 2
- Fetal risks include thyroid dysfunction and growth restriction 1
Postpartum Considerations
- Both PTU and methimazole are safe during breastfeeding 2, 9
- Methimazole is the preferred choice in lactating women 9
- Monitor for postpartum thyroiditis in women with history of thyroid dysfunction 2