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Introduction: To assess the relationship between parental consanguinity and permanent congenital hypothyroidism (PCH). Materials And Methods: From February 1998 to August 2002, cord dried blood spot samples, viz. only live births, in eight hospitals and a rural birth center in Tehran and Damavand were collected on Whatman BFC 180 filter papers. Samples with cord TSH  20 mU/L (two-site IRMA) were recalled. Between 7-14 days of life or thereafter, congenital hypothyroidism (CH) was confirmed by serum TSH > 10 mU/L and T4 < 6.5 g/dL and L-T4 was immediately started. CH-affected newborns were followed-up until May 2003. Thyroid dysgenesis was determined using 99m technetium pertechnetate thyroid scanning and/or ultrasonography. In thyroid eutopic newborns, dyshormonogenesis was diagnosed by a 4-week discontinuation of L-T4 between 2-3 years of age followed by abnormal serum TSH and T4 values. Results: Of 35067 specimens, 373 (1.06%) were recalled, 25 had PCH (1:1403 births), 18 had thyroid dysgenesis (1:1948 births), and 7 had thyroid dyshormonogenesis (1:5010 births). Twenty-one PCH cases were detected from July 2000 onwards and 6648 (28.6%) of 23227 neonates had parental consanguinity. The odds ratio of parental consanguinity in PCH was 2.75 (95%CI: 1.17-6.47 P=0.02) and in dysgenesis was 3.74 (95%CI: 1.33-10.52 P=0.01) and the odds ratio of first-cousin parental consanguinity in PCH was 2.96 (95%CI: 1.23-7.15 P=0.02) and in dysgenesis was 3.21 (95%CI: 1.14-9.02 P=0.03) compared to non-PCH and non-dysgenetic cases, respectively. Conclusions: Parental consanguinity can be considered as the possible causative factor for the high prevalence of PCH and dysgenesis in Tehran.