What is the best next step in managing a pregnant lady in her 36th week with hyperthyroidism, taking methimazole (Thiamazole) 5mg, with a Thyroid-Stimulating Hormone (TSH) level of 0.01 and elevated total T4 of 13.3?

Management Recommendation

Continue methimazole 5mg without dose adjustment and monitor thyroid function every 2-4 weeks until delivery. 1

Rationale for Current Management

At 36 weeks gestation (third trimester), methimazole is the preferred antithyroid medication, and the current clinical picture indicates adequate disease control:

• Methimazole is specifically preferred in the second and third trimesters over propylthiouracil (PTU), which is reserved for first trimester use due to lower teratogenic risk during organogenesis 1
• The patient's TSH of 0.01 (suppressed) with total T4 of 13.3 suggests she is in a subclinical hyperthyroid state, which is actually the therapeutic target during pregnancy 2, 1
• The treatment goal is to maintain free T4 or free thyroxine index (FTI) in the high-normal range using the lowest possible thioamide dosage 2, 1
• A suppressed TSH with controlled T4 levels indicates the current 5mg dose is appropriate and should not be increased 2

Key Monitoring Strategy

Continue regular thyroid function monitoring:

• Check free T4 or FTI every 2-4 weeks to guide any necessary dosage adjustments 2, 1
• The finding of a rising serum TSH would indicate that a lower maintenance dose should be employed 3
• Monitor for symptoms of agranulocytosis (sore throat, fever) which requires immediate discontinuation and complete blood count 2, 3

Critical Third Trimester Considerations

Fetal and neonatal monitoring requirements:

• Inform the newborn's physician about maternal Graves' disease due to risk of neonatal thyroid dysfunction 2, 1
• Monitor the mother for normal heart rate and appropriate fetal growth 2
• Ultrasound screening for fetal goiter is not necessary unless problems are detected 2
• Transient suppression of fetal/neonatal thyroid function can occur with thioamide therapy but usually resolves without treatment 2

Common Pitfalls to Avoid

Do not over-treat based on suppressed TSH alone:

• A suppressed TSH (0.01) in the setting of controlled T4 levels does not warrant dose escalation 2
• Over-treatment risks fetal hypothyroidism and goiter, which are more harmful than maternal subclinical hyperthyroidism 3
• Methimazole crosses placental membranes and can cause fetal goiter and cretinism if excessive doses are used 3

Do not switch to PTU at this late stage:

• PTU carries significant hepatotoxicity risk and is only preferred in first trimester 1, 4
• Switching medications in the third trimester introduces unnecessary risk without benefit 1

Postpartum Planning

Anticipate disease changes after delivery:

• Graves' disease often remits in late pregnancy but commonly relapses postpartum 5
• Breastfeeding is safe with methimazole at this dose 2, 3
• Continue monitoring thyroid function postpartum with potential need for dose adjustment 2