How to manage hyperthyroidism in a pregnant woman?

Management of Hyperthyroidism in Pregnancy

Use propylthiouracil (PTU) during the first trimester, then switch to methimazole for the second and third trimesters to minimize both birth defects and maternal hepatotoxicity. 1

Medication Selection by Trimester

First Trimester (Weeks 1-13)

• PTU is the preferred antithyroid drug during organogenesis because methimazole carries a risk of congenital malformations when used during weeks 6-10 of pregnancy 1, 2, 3
• Methimazole-associated birth defects can be severe, making PTU the safer choice despite its hepatotoxicity risk 1, 2

Second and Third Trimesters (Weeks 14-40)

• Switch from PTU to methimazole after the first trimester to reduce the risk of maternal hepatotoxicity 1
• Continuing PTU beyond the first trimester unnecessarily exposes the mother to liver injury risk 1
• The FDA label acknowledges that switching to methimazole for the second and third trimesters is preferable given PTU's hepatotoxic potential 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 dosage 1
• Aim for subclinical hyperthyroidism rather than complete normalization to minimize fetal exposure to antithyroid drugs 4
• Use free T4 and free T3 measurements rather than total hormone levels, as total T4 and T3 are elevated in normal pregnancy due to increased thyroxine-binding globulin 5

Monitoring Frequency

• Check FT4 or FTI every 2-4 weeks to guide dosage adjustments 1
• A rising TSH indicates the need for a lower maintenance dose 2
• Both maternal and fetal thyroid function require careful monitoring since antithyroid drugs, thyroid-stimulating antibodies, and thyroid hormones all cross the placenta 5, 3

Symptomatic Management

• Initiate beta-adrenergic blockers (such as propranolol) for symptomatic relief while awaiting thyroid control 6
• Beta-blockers readily cross the placenta, so use the lowest effective dose 5
• Dose reduction of beta-blockers may be needed as the patient becomes euthyroid due to decreased clearance 2

Risks of Untreated Hyperthyroidism

• Untreated maternal hyperthyroidism significantly increases risks including severe preeclampsia, preterm delivery, heart failure, miscarriage, stillbirth, and low birth weight 1, 2, 7
• Maternal thyrotoxicosis poses greater risks than the side effects of antithyroid drugs, making treatment essential 7, 5

Critical Monitoring for Drug Toxicity

• Immediately discontinue the thioamide if sore throat and fever develop, as this may indicate agranulocytosis 6
• Monitor for hepatitis, vasculitis, and thrombocytopenia 6
• Check prothrombin time before surgical procedures, as methimazole may cause hypoprothrombinemia 2

Fetal and Neonatal Considerations

• Inform the newborn's physician about maternal thyroid disease due to risk of neonatal thyroid dysfunction from transplacental passage of thyroid-stimulating antibodies or excessive antithyroid drug exposure 1, 7
• The fetal thyroid becomes fully responsive to both thyroid-stimulating antibodies and antithyroid drugs by 20 weeks' gestation 5
• Fetal/neonatal hyperthyroidism can occur in offspring of mothers with Graves' disease, requiring collaboration between endocrinologists, obstetricians, and neonatologists 7, 8

Postpartum Management

• Women treated with PTU or methimazole can safely breastfeed, as studies of 139 thyrotoxic lactating mothers found no toxicity in nursing infants 1, 2
• Monitor thyroid function at frequent (weekly or biweekly) intervals in breastfeeding mothers 2
• Thyroid dysfunction often diminishes as pregnancy progresses, and dose reduction or discontinuation may be possible weeks to months before delivery 2, 8

Common Pitfalls to Avoid

• Failing to switch from PTU to methimazole after the first trimester increases unnecessary hepatotoxicity risk 1
• Inadequate monitoring of thyroid function during pregnancy results in suboptimal treatment 1
• Over-treatment with antithyroid drugs can cause fetal hypothyroidism and goiter, so use the minimum effective dose 5, 3