This is really important info! points to remember:

"We have considered the possibility that the inclusion of cases of transient hypothyroidism has inflated the reported incidence rate of CH(congenital hypothyroidism)."

"Assessing the effects of cases oftransient hypothyroidism on the incidence rate is problematic, because the definitions, diagnostic criteria, and differentiation from transient hyperthyrotropinemia vary widely among state newbornscreening programs."

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http://pediatrics.aappublications.org/cgi/content/abstract/125/Supplement_2/S54

The Impact of TransientHypothyroidism on the Increasing Rate of Congenital Hypothyroidismin the United States

^a Division of Pediatric Endocrinology, Department of Pediatrics, Emory University School of Medicine, Atlanta, Georgia; 
^b National Center on Birth Defects and Developmental Disabilities, Centers for Disease Control and Prevention, Atlanta, Georgia; 
^c Department of State Health Services, Austin, Texas; 
^d Cook Children's Physician Network, Fort Worth, Texas; and 
^e Department of Epidemiology, Rollins School of Pubic Health, Emory University, Atlanta, Georgia

The reported incidence rate of primary congenitalhypothyroidism (CH) has been increasing in the United States over the past 2 decades. We have considered the possibility that the inclusion of cases of transienthypothyroidism has inflated the reported incidence rate of CH. Assessing the effects of cases oftransient hypothyroidism on the incidence rate is problematic, because the definitions, diagnostic criteria, and differentiation from transient hyperthyrotropinemia vary widely among state newbornscreening programs. Among the 4 etiologies for transient hypothyroidism (maternal thyrotropin receptor–blocking antibodies, exposure to maternal antithyroid medications, iodine deficiency, andiodine excess), there is little evidence of increases in the incidence rate from thyrotropin receptor–blocking antibodies. Exposure to antithyroid drugs could contribute significantly to the incidence rate oftransient CH, given the high estimated incidence of active maternal hyperthyroidism. Iodine deficiencyor excess in the United States seems unlikely to have contributed significantly to the incidence rate of CH, because the secular trend toward lower iodine intake among women of reproductive age in the 1980s and 1990s seems to have plateaued, and perinatal iodine exposure has presumably declined as a result of recommendations to discontinue using iodine-containing disinfectants. Although the female-to-male sex ratio among newborns with thyroid agenesis or dysgenesis (the most common causes of CH) is typically 2:1, analysis of the sex ratio of newborns diagnosed with presumed CH in the United States suggests that a substantial proportion might have transient hypothyroidism or hyperthyrotropinemia,because the sex ratio has been well below the expected 2:1 ratio. Combined ultrasonography and ¹²³I scintigraphy of the thyroid gland are effective tools for identifying cases of thyroid agenesis and dysgenesis and can help to differentiate cases of transient hypothyroidism from true CH. Imaging is also a vital component in evaluating children who, at 3 years of age, undergo a trial of discontinuation of levothyroxine treatment to test for persistence of hypothyroidism. Ultimately, thyroid gland imaging, in conjunction with long-term follow-up studies that appropriately assess and report whether there was permanence of hypothyroidism, will be necessary to address the true incidence rate of CH and anycontribution to the observed rate by transient cases of hypothyroidism or hyperthyrotropinemia.

Key Words: transient hypothyroidism • hyperthyrotropinemia • thyrotropin receptor–blocking antibodies • antithyroid drugs • iodine • thyroid imaging

Abbreviations: CH = primary congenital hypothyroidism • NBS = newborn screening • TRBAb = thyrotropin receptor–blocking antibody • T4 = thyroxine • TSH = thyrotropin • NNSIS = National Newborn Screening Information System • UI = urinary iodine • NHANES = National Health and Nutrition Examination Survey • CI = confidence interval