A neonate is diagnosed with congenital hypothyroidism on newborn screening (elevated TSH, low free T4). Mother has Hashimoto's thyroiditis and was taking methimazole during pregnancy for Graves' disease that relapsed. Which is the MOST LIKELY cause of congenital hypothyroidism in this infant?

• A Transplacental transfer of maternal TSH receptor-stimulating antibodies (TRAb)
• B Thyroid dysgenesis (athyreosis or ectopic thyroid) as a de novo developmental defect
• C Iodine deficiency due to maternal dietary restriction
• D Transplacental passage of methimazole causing fetal thyroid suppression (transient hypothyroidism) ✓

Explanation

In this scenario, the mother is on methimazole (an antithyroid drug) for Graves' disease. Methimazole crosses the placenta and can suppress the fetal thyroid, causing transient hypothyroidism in the neonate. This is important because it is TRANSIENT and will resolve within a few weeks as the drug clears—but the neonate still requires treatment to prevent neurodevelopmental harm during this period. TRAb transfer from Graves' disease mothers typically causes neonatal hyperthyroidism, not hypothyroidism. Thyroid dysgenesis is the most common cause of permanent congenital hypothyroidism overall, but this clinical scenario specifically points to drug-induced transient CH.

Reference: Ghai Essential Pediatrics, 10th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.