How should hyperthyroidism be managed in a pregnant woman at 28 weeks gestation?

Management of Hyperthyroidism at 28 Weeks Gestation

At 28 weeks gestation (third trimester), methimazole should be used as the antithyroid drug of choice to treat hyperthyroidism, maintaining free T4 at or just above the upper limit of the nonpregnant reference range with monthly monitoring. 1

Medication Selection by Trimester

Since this patient is at 28 weeks (third trimester), the critical first-trimester teratogenic window has passed:

• Use methimazole (MMI) as the preferred antithyroid drug at this gestational age 1
• If the patient was started on propylthiouracil (PTU) in the first trimester, she should have been switched to methimazole at the beginning of the second trimester 1
• PTU is only preferred in the first trimester due to MMI's association with aplasia cutis and choanal/esophageal atresia during organogenesis 2, 1
• MMI is safer than PTU beyond the first trimester due to PTU's rare but potentially fatal hepatotoxicity 2, 1

Treatment Targets

Maintain thyroid hormones at specific pregnancy-adjusted levels:

• Free T4: Keep at or slightly above the upper limit of the nonpregnant reference range 1
• Total T4: Maintain at 1.5 times the upper limit of normal reference range 2
• TSH: Will remain suppressed; do not use as sole treatment guide 1
• Use the lowest possible antithyroid drug dose to minimize fetal hypothyroidism risk 1

Monitoring Protocol

• Assess thyroid function monthly and adjust antithyroid drug dose accordingly 1
• Monitor for signs of inadequate control: failure to gain weight, heat intolerance, excessive sweating, tachycardia beyond normal pregnancy 1

Critical Fetal Assessment at This Gestational Age

At 22-26 weeks (which this patient has passed), TSH receptor antibodies (TRAb) should have been measured to assess risk of fetal/neonatal thyrotoxicosis 1:

• If TRAb was elevated at 22-26 weeks, close fetal monitoring is required
• If not yet done, measure TRAb now to guide neonatal monitoring decisions
• TRAb crosses the placenta and can cause fetal hyperthyroidism even if mother is euthyroid on treatment 1

Important Caveats

Do NOT use block-replacement therapy (antithyroid drug plus levothyroxine) in pregnancy - this increases fetal hypothyroidism risk 1

Distinguish from gestational thyrotoxicosis: If this is hCG-mediated hyperthyroidism rather than Graves' disease, antithyroid drugs are not indicated 1. Key differentiators:

• Graves' disease: presence of goiter, ophthalmopathy, positive TRAb
• Gestational thyrotoxicosis: typically resolves by mid-pregnancy, no autoimmune features

Alternative Treatment Considerations

Thyroidectomy is contraindicated at 28 weeks gestation:

• Surgery should only be performed in the second trimester if absolutely necessary 1
• At 28 weeks (third trimester), there is increased risk of preterm labor (4.5-5.5%) 1
• Reserve for refractory cases unresponsive to medical management 3

Radioactive iodine is absolutely contraindicated during pregnancy 4, 5

Beta-blockers may be used for short-term symptomatic relief of tachycardia and tremor 1

Postpartum Planning

Prepare the patient that Graves' disease commonly relapses postpartum 1. Methimazole remains safe during breastfeeding at low doses 2, 1, 3.