What is the next step for a 7th month pregnant lady with hyperthyroidism on methimazole (antithyroid medication) with a Thyroid-Stimulating Hormone (TSH) level of 1.5 and Free Thyroxine (T4) level of 8.5?

Management of Hyperthyroidism in the Third Trimester of Pregnancy

Continue current methimazole therapy with close monitoring as the patient's thyroid function tests indicate adequate control with TSH 1.5 and T4 8.5. 1, 2

Current Status Assessment

• The patient's thyroid function tests (TSH 1.5, T4 8.5) indicate well-controlled hyperthyroidism on the current methimazole regimen 1, 2
• Methimazole is appropriate for second and third trimester treatment of hyperthyroidism in pregnancy 2, 3
• The goal of treatment is to maintain FT4 or FTI in the high-normal range using the lowest possible thioamide dosage 1, 2

Recommended Management Plan

• Continue current methimazole therapy as it appears to be effectively controlling thyroid function 2, 3
• Monitor FT4 or FTI every 2-4 weeks to guide any necessary dosage adjustments 1, 2
• Check TSH level once more before delivery 1, 2
• Maintain the lowest effective dose to minimize potential fetal exposure while controlling maternal disease 3

Monitoring Considerations

• Continue monitoring for potential side effects of methimazole therapy including:
  • Agranulocytosis (symptoms: fever, sore throat) 3
  • Hepatotoxicity (symptoms: anorexia, pruritus, right upper quadrant pain) 3
  • Vasculitis (symptoms: rash, hematuria, dyspnea) 3
• Monitor fetal growth and heart rate at regular prenatal visits 1, 2
• Routine ultrasound screening for fetal goiter is not necessary unless problems are detected 1, 2

Delivery Planning

• Inform the newborn's physician about maternal hyperthyroidism and methimazole treatment due to the associated risk of neonatal thyroid dysfunction 1, 2
• In many pregnant women, thyroid dysfunction diminishes as pregnancy proceeds; a reduction of dosage may be possible as delivery approaches 3
• In some cases, anti-thyroid therapy can be discontinued several weeks or months before delivery 3

Postpartum Considerations

• Women treated with methimazole can breastfeed safely 1, 2
• Several studies have found no effect on clinical status in nursing infants of mothers taking methimazole 3
• Continue monitoring thyroid function at frequent intervals during breastfeeding 3

Important Cautions

• Untreated or inadequately treated maternal hyperthyroidism increases risks of preeclampsia, preterm delivery, heart failure, and low birth weight 3, 2
• Avoid radioactive iodine (I-131) as it is absolutely contraindicated during pregnancy 2
• Thyroidectomy should be reserved only for women who do not respond to thioamide therapy 1, 2