When should neonatal screening for congenital hypothyroidism be performed?

• A. Newborn at least 3 days old
• B. At birth
• C. At 1 month
• D. At 6 weeks

Answer: (A)

Timing of neonatal screening for congenital hypothyroidism hinges on the baby’s thyroid status stabilizing after birth. Right after delivery, maternal thyroid hormones cross the placenta and the newborn’s own thyroid axis is immature, with a transient TSH surge in the first day or two. If screening is done too early, results can be unreliable—false positives from the surge or false negatives if the newborn’s thyroid hasn’t yet produced enough hormone. Waiting until at least three days of life gives time for these factors to settle, making abnormal results more indicative of true thyroid dysfunction. Early detection and treatment with thyroid hormone are essential to prevent neurodevelopmental impairment, so screening at or after about three days is the most reliable approach. Screening at birth, at one month, or at six weeks could miss early cases or delay therapy.