Newborn screening for Congenital Hypothyroidism (CH) is a major public health achievement. Thyroid hormone is essential for the maturation of brain function and somatic growth, and its deficiency early in life can lead to mental retardation. For the fetus, maternal thyroid status is important during the first half of gestation; thereafter, the fetus’ hypothalamus-pituitary-thyroid axis is functional in the normal situation. For the hypothyroid newborn, it is well documented that provision of thyroid hormone is critical during the first weeks of life to avoid severe intellectual impairment. Notably, congenital hypothyroidism is considered one of the most common preventable causes of mental retardation.
Studies showed that affected newborns were rarely identified during the first months of life and were often missed until 1-3 years of age. Congenital hypothyroidism was found to be an ideal candidate with the introduction of dried blood newborn screening by Dussault in Canada. With the development of increasingly sensitive assays to measure thyroid hormone (T4) and thyroid stimulating hormone (TSH) using a dried blood spot (DBS), newborn screening programs have developed throughout much of the world. In the 1980s, the incidence of CH in the United States was estimated to be 1:3000-1:4000. More recently, screening programs have reported an increased incidence of 1:1400-1:2800, most probably due to changes in screening strategies and the identification of milder cases.
Typically, newborn screening requires a heel stick blood specimen obtained at 48-72 hrs of life prior to an infant’s discharge from the hospital. Most current assays measure TSH alone as an indicator of thyroid function. Results above established cutoff levels generally signify thyroid gland dysfunction and indicate further testing. Although most helpful in early identification of term newborns with anatomic or functional thyroid gland abnormalities, the screening does miss a percentage of newborns, for example those with central hypothyroidism due to hypothalamic-pituitary failure and the increasingly larger group of preterm infants with congenital hypothyroidism who demonstrate delayed elevations in TSH. Numerous questions remain regarding the optimal timing of follow up laboratory studies and even treatment of certain types of newborn thyroid dysfunction.Nevertheless, newborn screening has proved invaluable for the great number of affected newborns.
The American Academy of Pediatrics recommends the measurement of TSH in all newborns with the goal that all infants with CH be identified by 2 weeks of age and that effective treatment with thyroid hormone replacement be started such that serum TSH levels less than 5 mIU/L be achieved within 4 weeks of diagnosis. Unfortunately, despite the significant successes following early identification and treatment of newborns with CH, obstacles persist in reaching the Academy’s goals. Screening programs continue to be plagued with the practical problems of screening all newborns, particularly those discharged home early who are lost to recall or lost to follow up altogether. In addition, dried blood specimens are collected or processed improperly. Delays occur with recall of infants with abnormal results and with appropriate referrals for definitive treatment and management. A recent study conducted in Utah and reported at the 86th Annual Meeting of the American Thyroid Association highlights some of the problems which currently exist. After reviewing the TSH assays of 4394 children under 2 years of age, 48% of initial samples with elevated levels (>20 mIU/L) were obtained after the first 2 weeks of life, 15% of the initial abnormal TSH assays were not retested, and only 34% of those infants with initial elevated TSH assays achieved the goal of TSH < 5 mIU/L within 28 days of the initial assay.
The final message is that it is not enough to rely on the known efficacy of newborn screening for congenital hypothyroidism, but greater vigilance must be exercised to maximize its benefits in the lives of children.