In a term pregnant woman in active labor with known diffuse toxic goiter and suppressed thyroid‑stimulating hormone with high‑normal free triiodothyronine and free thyroxine, can antithyroid therapy be started with methimazole?

Can Methimazole Be Started in This Term Pregnant Woman in Active Labor?

No, methimazole should not be started in this patient who is in active labor at term. At this stage of pregnancy, the immediate priority is safe delivery, and any antithyroid medication started now will not meaningfully impact maternal or fetal outcomes before delivery occurs. 1

Rationale for Withholding Methimazole at This Time

Current Clinical Status Does Not Require Immediate Antithyroid Therapy

• The patient is clinically stable with no signs of thyroid storm: no fever, no tachycardia (heart rate 85), no altered mental status, no tremors, no palpitations, and hemodynamically stable (BP 90/60). 2

• Thyroid function tests show biochemical control: TSH is suppressed (<0.01) as expected in diffuse toxic goiter, but free T3 (3.36, reference 2.77-5.27) and free T4 (1.38, reference 0.78-2.19) are both within normal limits. This indicates the patient is not overtly thyrotoxic at present. 1

• Thyroid storm is not present, which would be the only emergency indication for immediate antithyroid drug therapy during labor. Thyroid storm requires immediate treatment with propylthiouracil or methimazole, saturated potassium iodide, dexamethasone, and supportive measures—but this patient has none of the clinical features (fever, severe tachycardia, altered mental status, cardiovascular instability). 2, 1

Delivery Should Proceed Without Delay

• The patient is in active labor at term, and delivery should not be delayed for initiation of antithyroid therapy unless thyroid storm is present. 2

• Starting methimazole now will not provide benefit before delivery because antithyroid drugs require days to weeks to achieve meaningful reduction in circulating thyroid hormone levels. 1, 3

Methimazole-Specific Concerns in Late Pregnancy

• Methimazole crosses the placenta and can induce fetal goiter and hypothyroidism, though this risk is primarily relevant with chronic exposure rather than a single dose at term. 4, 5

• The fetal thyroid is fully responsive to antithyroid drugs by 20 weeks' gestation, meaning any methimazole given now would affect the fetus, but delivery is imminent so this exposure would be brief. 5

Post-Delivery Management Plan

Immediate Post-Partum Period

• Restart methimazole after delivery if the patient requires ongoing treatment for diffuse toxic goiter. The decision to restart should be based on repeat thyroid function tests obtained 1-2 weeks postpartum. 1

• Methimazole is compatible with breastfeeding. Multiple studies have shown no adverse effects on nursing infants when mothers take methimazole, with monitoring of infant thyroid function recommended. 4, 3

• Monitor for postpartum thyroiditis, which can occur in women with a history of thyroid dysfunction. New onset of abnormal TSH or free T4 after delivery warrants evaluation. 2, 1

Long-Term Antithyroid Drug Strategy

• If antithyroid therapy is needed postpartum, methimazole is the preferred agent for non-pregnant women due to lower risk of hepatotoxicity compared to propylthiouracil. 1, 3

• The goal of therapy is to maintain free T4 in the upper one-third of the reference range using the lowest effective dose to minimize risk of maternal hypothyroidism. 3

• Monitor thyroid function every 2-4 weeks initially until stable, then every trimester if the patient becomes pregnant again. 1

Critical Pitfalls to Avoid

• Do not delay delivery to start antithyroid medication unless thyroid storm is present. The patient is clinically stable and delivery should proceed. 2

• Do not assume all patients with suppressed TSH require immediate antithyroid therapy. This patient's free T3 and free T4 are normal, indicating biochemical control despite TSH suppression. 1

• Do not use radioactive iodine (I-131) during pregnancy or within 4 months of breastfeeding, as it causes fetal thyroid ablation and is absolutely contraindicated. 1, 4

• Inform the newborn's physician about maternal thyroid disease due to risk of neonatal thyroid dysfunction from transplacental passage of thyroid-stimulating immunoglobulins in Graves' disease. 1, 3

If Thyroid Storm Were Present (It Is Not)

• Thyroid storm is a medical emergency requiring immediate treatment without waiting for laboratory confirmation. 2, 1

• Standard therapy includes: propylthiouracil or methimazole, saturated potassium iodide or sodium iodide, dexamethasone, phenobarbital, and supportive measures (oxygen, antipyretics, appropriate monitoring). 2

• Delivery during thyroid storm should be avoided unless deemed absolutely necessary, as maternal and fetal outcomes are significantly worse. 2

Neonatal Considerations

• The neonate should be evaluated for thyroid dysfunction given maternal history of diffuse toxic goiter and previous methimazole exposure during this pregnancy. 1, 3

• Neonatal hyperthyroidism or hypothyroidism can occur due to transplacental passage of maternal thyroid antibodies or antithyroid drugs. 3, 6

• Cord blood or neonatal thyroid function tests (TSH, free T4) should be obtained within the first few days of life. 1