Congenital hypothyroidism, previously known as cretinism, is a severe deficiency of thyroid hormone in newborns. It causes impaired neurological function, stunted growth, and physical deformities. The condition may occur because of a problem with the baby’s thyroid gland, or a lack of iodine in the mother’s body during pregnancy.

A baby’s body needs iodine to make thyroid hormones. These hormones are essential for healthy growth, brain, and nervous system development.

Between 1 in 2,000 and 1 in 4,000 babies are born with congenital hypothyroidism.

The introduction of iodized salt in the early 20th century made congenital hypothyroidism very rare in the United States and the rest of the Western world. However, severe iodine deficiency is still common in developing nations.

Myxedema is a term used to describe a severely underactive thyroid gland in an adult. Congenital hypothyroidism refers to a thyroid deficiency in an infant.

Myxedema can also be used to describe skin changes caused by low thyroid hormone levels.

Signs of cretinism or congenital hypothyroidism in a newborn include:

  • lack of weight gain
  • stunted growth
  • fatigue, lethargy
  • poor feeding
  • thickened facial features
  • abnormal bone growth
  • mental retardation
  • very little crying
  • excessive sleep
  • constipation
  • yellowing of the skin and whites of the eyes (jaundice)
  • floppiness, low muscle tone
  • hoarse voice
  • unusually large tongue
  • swelling near the navel (umbilical hernia)
  • cool, dry skin
  • pale skin
  • swelling of the skin (myxedema)
  • swelling in the neck from an enlarged thyroid gland (goiter)

Congenital hypothyroidism in newborns can be caused by:

  • a missing, poorly formed, or abnormally small thyroid gland
  • a genetic defect that affects thyroid hormone production
  • too little iodine in the mother’s diet during pregnancy
  • radioactive iodine or antithyroid treatment for thyroid cancer during pregnancy
  • use of medicines that disrupt thyroid hormone production — such as antithyroid drugs, sulfonamides, or lithium — during pregnancy

Iodine deficiency is no longer considered a health risk in the United States due to the introduction of iodized salt. However, it’s still the most common preventable cause of impaired neurological function in the world.

Because our bodies don’t make iodine, we need to get it from food. Iodine gets into food through soil. In some parts of the world, the soil is lacking in iodine.

Newborns in the United States and many other countries are routinely screened for thyroid hormone levels. The test involves taking a small blood sample from the baby’s heel. A laboratory checks the baby’s blood levels of thyroid hormone (T4) and thyroid-stimulating hormone (TSH).

Doctors called pediatric endocrinologists treat congenital hypothyroidism. The main treatment is to give the baby thyroid hormone (levothyroxine). This condition must be treated within the first four weeks after birth or the intellectual disability may be permanent.

Thyroid hormone comes in a pill that parents can crush up into their baby’s breast milk, formula, or water. Parents need to be cautious about using some formulas. Soy protein and concentrated iron formulas can interfere with the absorption of thyroid hormone.

Once babies are on thyroid hormone medication, they will need to have blood tests every few months. These tests will check that their TSH and T4 levels are within a normal range.

Congenital hypothyroidism is typically seen in developing countries where iodine deficiency is common. Adults can prevent iodine deficiency by getting the Institute of Medicine’s recommended dietary allowance (RDA) of 150 micrograms of iodine per day. One teaspoon of iodized salt contains about 400 micrograms of iodine.

Because an iodine deficiency in pregnancy can be dangerous to the growing baby, pregnant women are advised to get 220 micrograms of iodine daily. The American Thyroid Association recommends that all women who are pregnant or breastfeeding take a prenatal vitamin containing at least 150 micrograms of iodine each day.

Children who are born with a severely underactive thyroid gland can develop intellectual disability if the condition isn’t treated quickly. A child’s IQ can drop several points for every few months that treatment is delayed. Growth and bone strength can also be affected.

Other complications of congenital hypothyroidism include:

  • an abnormal walk
  • muscle spasticity
  • an inability to speak (mutism)
  • autistic behavior
  • vision and hearing problems
  • problems with memory and attention

Even with treatment, some children with congenital hypothyroidism may be slower to learn than other kids their age.

The outlook depends on how quickly a baby is diagnosed and treated. Infants who aren’t diagnosed or treated within the first few weeks after birth have a lower IQ and more physical health problems than those who are treated sooner.