Newborn screening · Endocrine disorder
Primary Congenital Hypothyroidism · CH
The most common condition detected by newborn screening. Levothyroxine fully prevents intellectual disability.
Description
Primary congenital hypothyroidism is inadequate thyroid hormone production present from birth. The thyroid hormones thyroxine (T4) and triiodothyronine (T3) drive brain development in the first years of life, and untreated congenital hypothyroidism causes severe, permanent intellectual disability. Early treatment prevents that outcome, which is why congenital hypothyroidism is among the founding conditions of newborn screening worldwide.
Two broad mechanisms account for primary disease. Thyroid dysgenesis, an absent, ectopic, or hypoplastic gland, causes roughly 80 to 85 percent of permanent cases and is usually sporadic. Thyroid dyshormonogenesis, an inherited defect in hormone synthesis, accounts for roughly 15 to 20 percent and is most often autosomal recessive. Genes implicated in dyshormonogenesis include TG, TPO, DUOX2, DUOXA2, SLC5A5, SLC26A4, and IYD. Genes implicated in dysgenesis include TSHR, PAX8, NKX2-1, NKX2-5, and FOXE1, although most dysgenesis cases have no identified genetic cause. Central congenital hypothyroidism, caused by pituitary or hypothalamic disease, is a separate, less common category.
Affected newborns are typically asymptomatic at birth. Classic clinical signs, including prolonged jaundice, poor feeding, hypotonia, large fontanelles, macroglossia, umbilical hernia, and constipation, develop over weeks if treatment is delayed. By that point, neurodevelopmental injury is already underway.
Detection is by newborn screening on the dried blood spot, using either primary thyroid-stimulating hormone (TSH), primary thyroxine with reflex TSH, or both, depending on state or country protocol. Confirmation uses serum TSH and free T4, with thyroid ultrasound or radionuclide scintigraphy to clarify mechanism when desired. Newborn screening for congenital hypothyroidism was developed by Jean Dussault and colleagues in Quebec in the early 1970s, with the first pilot reported by Dussault and Laberge (Union Médicale du Canada, 1973).
Reported worldwide birth incidence of primary congenital hypothyroidism is approximately 1 in 2,000 to 1 in 4,000, varying with screening cutoffs and inclusion of milder cases. Congenital hypothyroidism is the most common condition detected by newborn screening in the United States.
Treatments to date
Treatment begins as soon as primary congenital hypothyroidism is confirmed after a positive newborn screen, ideally within the first two weeks of life. The goal is to normalize thyroid hormone levels quickly and maintain them through the period of rapid brain development and beyond.
Oral levothyroxine, synthetic L-thyroxine, is the standard of care. The American Academy of Pediatrics and the European Society for Paediatric Endocrinology recommend a starting dose of 10 to 15 micrograms per kilogram per day, with the higher end of the range used for severe biochemical disease. Tablets are crushed and given with a small volume of water or breast milk; soy formula, iron, and calcium reduce absorption and are separated from the dose. Serum TSH and free T4 are checked at intervals through infancy and childhood, with dose adjustments tied to growth and laboratory targets.
Treatment is lifelong for permanent congenital hypothyroidism. Transient cases, more common in preterm infants, in iodine-exposed infants, and in some families with maternal thyroid antibodies, can be identified by a supervised trial off therapy after age three, when brain development is no longer at risk.
Outcomes with early, adequate levothyroxine are substantially normal. Long-term cohort studies from screening programs in North America and Europe report intelligence quotients within population norms when treatment starts in the first weeks at adequate dose, with subtle deficits more common when treatment is delayed or undertreated in severe disease.
No disease-modifying gene therapy exists for primary congenital hypothyroidism, and none is in clinical development.