What is the best approach to manage hyperthyroidism (overactive thyroid) in a pregnant female?

Management of Hyperthyroidism in Pregnancy

Use propylthiouracil (PTU) as first-line treatment during the first trimester, then switch to methimazole for the second and third trimesters to minimize both teratogenic risk and maternal hepatotoxicity. 1, 2

Medication Selection by Trimester

First Trimester (Weeks 1-13)

• PTU is the preferred antithyroid drug during organogenesis because methimazole carries higher risk of congenital abnormalities including aplasia cutis, choanal atresia, and esophageal atresia 1, 2, 3
• Start PTU immediately upon diagnosis to prevent maternal complications (preeclampsia, heart failure, preterm delivery) and fetal complications (low birth weight, stillbirth) 1, 2

Second and Third Trimesters (Weeks 14-40)

• Switch from PTU to methimazole after the first trimester to reduce risk of PTU-associated hepatotoxicity 1, 2
• Methimazole becomes the safer option once organogenesis is complete 2, 3
• Failure to make this switch increases maternal risk of severe liver injury 2

Treatment Goals and Monitoring

Target Thyroid Levels

• Maintain free T4 or Free Thyroxine Index (FTI) in the high-normal range using the lowest possible thioamide dose 1, 2
• This approach minimizes fetal exposure while adequately treating maternal hyperthyroidism 1

Monitoring Schedule

• Monitor free T4 or FTI every 2-4 weeks to guide dosage adjustments 1, 2
• Check TSH every trimester for overall stability 1
• Many pregnant women experience spontaneous improvement in hyperthyroidism as pregnancy progresses, allowing dose reduction or even discontinuation weeks before delivery 3

Adjunctive Symptomatic Management

• Beta-blockers (propranolol) can be used temporarily for symptom control (tremors, palpitations, tachycardia) until thioamide therapy reduces thyroid hormone levels 1
• Use beta-blockers judiciously and discontinue once euthyroid state is achieved, as hyperthyroidism increases beta-blocker clearance 3

Surgical Management

Indications for Thyroidectomy

• Reserve surgery for women who fail thioamide therapy or develop drug intolerance (agranulocytosis, severe hepatotoxicity) 1
• If surgery is necessary, perform during the second trimester when surgical risk is lowest 1

Absolute Contraindications

• Radioactive iodine (I-131) is absolutely contraindicated during pregnancy as it causes fetal thyroid ablation 1
• Women must wait 4 months after I-131 treatment before attempting pregnancy or breastfeeding 1

Critical Side Effects to Monitor

Agranulocytosis

• Immediately discontinue thioamide if patient develops sore throat and fever, which indicate potential agranulocytosis 1
• This is a life-threatening complication requiring urgent evaluation 4

Other Serious Adverse Effects

• Monitor for hepatitis, vasculitis, and thrombocytopenia throughout treatment 4
• PTU carries higher hepatotoxicity risk, particularly in the second and third trimesters, reinforcing the need to switch to methimazole 2

Special Clinical Scenarios

Hyperemesis Gravidarum

• Biochemical hyperthyroidism associated with hyperemesis gravidarum rarely requires antithyroid treatment unless other clinical signs of hyperthyroidism are present 1
• This is typically hCG-mediated and self-limited 1

Thyroid Storm

• Thyroid storm is a medical emergency presenting with fever, disproportionate tachycardia, altered mental status, vomiting, diarrhea, and cardiac arrhythmia 1
• Begin treatment immediately without waiting for laboratory confirmation 1
• Treatment includes PTU or methimazole, potassium/sodium iodide solutions, dexamethasone, phenobarbital, and supportive care 1
• Avoid delivery during thyroid storm unless absolutely necessary 1

Neonatal Considerations

• Inform the newborn's physician about maternal Graves' disease due to risk of neonatal thyroid dysfunction from transplacental passage of thyroid-stimulating antibodies 1, 2
• Fetal thyroid becomes fully responsive to maternal antibodies and antithyroid drugs by 20 weeks gestation 5
• Women treated with PTU or methimazole can safely breastfeed, as studies show no adverse effects on nursing infants with appropriate monitoring 2, 3

Common Pitfalls to Avoid

• Not switching from PTU to methimazole after the first trimester exposes the mother to unnecessary hepatotoxicity risk 2
• Failure to monitor thyroid function every 2-4 weeks may result in overtreatment causing fetal hypothyroidism and goiter, or undertreatment causing maternal and fetal complications 1, 2
• Overtreating hyperthyroidism can induce fetal hypothyroidism since antithyroid drugs cross the placenta 3, 5