A condition of thyroid hormone deficiency present at birth. Approximately 1 in 4000 newborn infants has a severe deficiency of thyroid function, while even more have mild or partial degrees. If untreated for several months after birth, congenital hypothyroidism can lead to intellectual disability such as permanent mental retardation and abnormal growth. In the United States and many other countries, all newborns are tested for congenital hypothyroidism. If treatment begins in the first month after birth, infants usually develop normally.

Treatment usually consists of a daily dose of thyroid hormone (thyroxine) by mouth. Because the treatment is simple, effective, and inexpensive, nearly all of the developed world practises newborn screening to detect and treat congenital hypothyroidism in the first weeks of life.