The screening test for congenital hypothyroidism in newborns is based on which parameter?

• A. TSH Level in the Newborn
• B. Thyroid-Binding Globulin Level in the Newborn
• C. Iodine Level in the Newborn
• D. Total Thyroxine (T4) Level in the Newborn

Answer: (D)

Detecting congenital hypothyroidism early relies on identifying low thyroid hormone in the newborn, because a deficiency during the critical period of brain development can cause lifelong impairments if not treated promptly. Total T4 provides a direct measure of the circulating thyroid hormone available to the tissues at birth; in infants with hypothyroidism, this level is typically reduced. Practical screening on dried blood spots uses total T4 because it is a stable, reliably measurable marker in this format, enabling a broad and efficient initial screen. If a low T4 is found, a follow-up test measuring TSH helps confirm the diagnosis and distinguishes primary thyroid failure from other causes, guiding treatment decisions. Other markers, like thyroid-binding globulin or iodine levels, do not serve as effective initial screening tools for detecting congenital hypothyroidism.