Every year in the United States, approximately 1 in 4,000 babies is born with congenital hypothyroidism. The term congenital simply means that the condition is present at birth. A baby who is born with hypothyroidism is at serious risk for mental retardation and growth problems. The good news is that since the 1970s, most industrialized countries, including the United States and Canada, have screened newborns for thyroid disease with a simple blood test that has helped to significantly reduce the risk for retardation in babies born with hypothyroidism.
For this test, a drop of blood is drawn from your baby's heel about twenty-four to seventy-two hours after birth. The blood is placed on a special filter paper that can measure your baby's TSH levels. If TSH is high, your physician will immediately start treating your baby with levothyroxine to replace the missing thyroid hormone that her body isn't naturally producing. If TSH is slightly elevated, your doctor may test your baby's free T4 levels first before starting treatment.
Some doctors may also do an X-ray of your baby's legs to look at the ends of the bones, which may appear immature in babies with congenital hypothyroidism. Doctors may also do a thyroid scan to determine the location or absence of the thyroid gland (see below).
According to the March of Dimes, congenital hypothyroidism is the most common disorder identified by screening in newborns. Other diseases include phenylketonuria (PKU), in which the baby can't process a part of an amino acid called phenylalanine, and galactosemia, in which the baby can't convert a sugar called galactose into glucose. All three conditions can cause mental retardation. Incidentally, the same blood sample can screen for at least fifty-five different diseases.
Sometimes, follow-up blood tests will show that your baby's thyroid has resumed normal function. But because of the seriousness of untreated hypothyroidism, some doctors will opt to go ahead and continue treatment until future tests show that the child is euthyroid.
In rare cases, your baby's inadequate thyroid hormone may be a temporary problem called transient congenital hypothyroidism. This condition may occur if the mother was exposed to excess iodine during pregnancy, or if she took too much antithyroid medication for hyperthyroidism. In both instances, the baby's thyroid is temporarily blocked from producing enough thyroid hormone. An iodine deficiency can also cause transient congenital hypothyroidism. Even more rare, transient congenital hypothyroidism may be the result of excess amounts of TSI found in Graves' disease in the mother.
To give your baby levothyroxine, crush the tablet and dissolve it in water, then feed it with a dropper. Do not mix levothyroxine in soy-based infant formula; this can inhibit absorption of the drug.
If the hypothyroidism is transient, your baby's thyroid gland will correct itself over time, and thyroid hormone levels will eventually become normal. Transient congenital hypothyroidism is generally more common in premature or low-birth-weight babies. But it's impossible to know whether your child has transient or permanent hypothyroidism, which is why all babies who have low levels of TSH are treated with thyroid hormone replacement.