For the verification of birthdate as a risk factor, we considered retrospectively a data set of all consecutives newborns of Marche Region (Italy) affected by Congenital Hypothyroidism, in the period 1981-1996 (n = 92). For the evaluation of seasonal cyclic trends in TSH, we considered the level of the hormone in all newborns of Marche Region screened in the period 1989-1996. Data were collected from Neonatal Screening Register of Marche Region