Newborn screening for congenital hypothyroidism.

Most neonates born with congenital hypothyroidism (CH) have normal appearance and no detectable physical signs. Hypothyroidism in the newborn period is almost always overlooked and delayed diagnosis leads to the most severe outcome of CH, mental retardation, emphasizing the importance of neonatal screening. Blood spot T4 or TSH or both can be used in neonatal screening for CH. The latter, which is more sensitive, is not cost effective, so the first two are used in different programs in the world. TSH screening was shown to be more specific in the diagnosis of CH; T4 screening is more sensitive in detecting newborns especially with rare hypothalamic-pituitary hypothyroidism, but less specific with a high frequency of false positives mainly in low birth weight and premature infants. The time at which the sample is taken may vary between centers, with the majority taking blood from a heel prick after 24 hours of age to minimize the false positive high TSH due to the physiological neonatal TSH surge that elevates TSH levels and causes dynamic T4 and T3 changes in the first 1 or 2 days after birth. Early discharge of mothers postpartum has increased the ratio of false positive TSH elevations. Although transient hypothyroidism may occur frequently, all suspected infants should be treated as having CH for the first 3 years of life, taking into account the risks of mental retardation. A reevaluation after 3 years is needed in such patients. The goal of initial therapy in CH is to minimize neonatal central nervous system exposure to hypothyroidism by normalizing thyroid function, as reflected by T4 and TSH levels, as rapidly as possible. Iodine deficiency is the most important cause of CH worldwide. Iodine is essential for thyroid hormone synthesis and is present in soil, water and air. Prevention of iodine deficiency can be by iodized salt, iodized oil, iodized bread or iodine tablets.