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Congenital hypothyroidism (CH) screening was introduced in 1974 and has become common practice, preventing the devastating neurological and developmental outcomes of untreated CH. Thyroid hormones are essential for normal brain development during early years. It is uniformly agreed that screening programs are essential for early identification and treatment, and that adequate thyroxine (T4) dosing both initially and during the first three years of life correlates with good outcomes.1–3 There is, however, controversy regarding the timing of repeat thyroid function tests (TFTs) after the initial screen, as well as the frequency of monitoring required to optimize the outcomes of children diagnosed with CH who are being treated. This paper discusses current monitoring guidelines and reviews recent studies suggesting the need for changing these guidelines.
Incidence of Congenital Hypothyroidism
Before the use of newborn screening programs was widespread, the reported incidence of CH was 1:7,000–1:10,000. As expected, the CH incidence rate increased significantly—to approximately 1:4000—after the introduction of screening programs.4 Another significant increase in reported incidence has occurred during the last two decades. It is unclear whether this increase is the result of increased detection or a true increase in incidence due to a change in unidentified risk factors.
One possible reason for the higher reported incidence of CH is the change in screening methods and cut-offs used for its diagnosis. The use of a thyroid stimulating hormone (TSH) assay for initial screening is associated with a reported CH incidence higher by 24 % than that reported by laboratories using a T4 assay.5 During the 1990s, many laboratories switched to an initial TSH testing method, possibly contributing to the higher current measured incidence of CH. A recent paper by Hertzberg and colleagues, however, concluded that there was indeed an increased incidence rate, even after adjusting for the screening methodologies and parameters.5