How to manage a third trimester pregnant woman with hyperthyroidism, Thyroid Stimulating Hormone (TSH) level of 0.025, currently taking methimazole (Tapazole) 5 mg?

Management of Third Trimester Hyperthyroidism on Methimazole 5 mg with TSH 0.025

Continue methimazole 5 mg daily and measure free T4 (FT4) or free thyroxine index (FTI) immediately to guide dose adjustment, with the goal of maintaining FT4/FTI in the high-normal range using the lowest possible thioamide dosage. 1

Immediate Assessment

• Obtain FT4 or FTI levels now to determine if the patient is adequately controlled or over-treated, as TSH alone is insufficient for management decisions in pregnancy 1
• The suppressed TSH (0.025) could indicate either:
  • Appropriate control with FT4/FTI in high-normal range (goal state)
  • Over-treatment with FT4/FTI below normal range (requires dose reduction)
  • Under-treatment with FT4/FTI still elevated (requires dose increase)

Medication Management

Methimazole is appropriate to continue in the third trimester 1, 2:

• The FDA warning about congenital malformations applies specifically to first trimester exposure during organogenesis 2
• Recent studies show no significant differences between propylthiouracil and methimazole in fetal outcomes, with similar rates of fetal anomalies for both agents 1
• Given the potential maternal adverse effects of propylthiouracil (particularly hepatotoxicity), it is preferable to use methimazole for the second and third trimesters 2

Dosing Strategy

Adjust methimazole dose based on FT4/FTI results 1:

• Target: FT4 or FTI in the high-normal range (not mid-range or low-normal) 1
• Use the lowest possible thioamide dosage to achieve this target 1
• If FT4/FTI is already high-normal: continue current 5 mg dose
• If FT4/FTI is low-normal or below normal: reduce dose or consider temporary discontinuation
• If FT4/FTI remains elevated: increase dose cautiously

Monitoring Protocol

Measure FT4 or FTI every 2-4 weeks throughout the remainder of pregnancy 1:

• TSH remains suppressed for extended periods even after achieving euthyroidism, making it unreliable for dose titration 1
• Monitor maternal heart rate and ensure appropriate fetal growth 1
• Unless problems are detected, ultrasound screening for fetal goiter is not necessary 1

Fetal Considerations

Transient fetal/neonatal thyroid suppression may occur but rarely requires treatment 1:

• Although methimazole crosses placental membranes and can suppress fetal thyroid function, this is usually transient 1
• The risk of fetal hypothyroidism is minimized by maintaining maternal FT4/FTI in the high-normal range with the lowest effective dose 1, 2
• Alert the newborn's physician that the mother has hyperthyroidism due to the associated risk of neonatal thyroid dysfunction 1

Important Caveats

Watch for agranulocytosis warning signs 1:

• Instruct the patient to immediately report fever or sore throat 1, 2
• If these symptoms develop, obtain a complete blood count immediately and discontinue methimazole 1
• Other serious adverse effects include hepatitis, vasculitis, and thrombocytopenia 1, 2

Symptomatic management if needed 1:

• If the patient has persistent tachycardia or other hyperthyroid symptoms, add a beta blocker (e.g., propranolol) until thyroid hormone levels normalize 1

Breastfeeding counseling 1, 2:

• Women treated with methimazole can breastfeed safely 1
• Several studies found no effect on clinical status in nursing infants of mothers taking methimazole, with a long-term study of 139 thyrotoxic lactating mothers demonstrating no toxicity 2