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Congenital hypothyroidism occurs when an infant is born with a thyroid gland that is not producing enough thyroid hormones. Untreated congenital hypothyroidism may cause mental retardation.

Screening for this condition involves looking for a high level of the hormone TSH in the baby’s blood. Blood is normally sampled from the baby’s heel ('heelprick') and collected onto a piece of card (Guthrie card). Several spots of blood can be collected at the same time to allow screening for other conditions, such as phenylketonuria (PKU).

If the thyroid testing results indicate congenital hypothyroidism, thyroid hormone supplements may be necessary.