Context: Congenital hypothyroidism (CH) is the most common neonatal endocrine disorder, affecting one in 3000 to 4000 newborns. Since the introduction of a newborn screening program in 1988, more than 300 cases have been identified. The underlying genetic defects have not been systematically studied. Objective: To identify the mutation spectrum of CH-causing genes. Methods: Fifty-five patients from 47 families were studied by next-generation exome sequencing. Results: Mutations were identified in 52.7%of patients (29 of 55) in the following 11 genes: TG, TPO, DUOX2, SLC26A4, SLC26A7, TSHB, TSHR, NKX2-1, PAX8, CDCA8, and HOXB3. Among 30 patients with thyroid dyshormonogenesis, biallelic TG mutations were found in 12 patients (40%), followed by biallelicmutations in TPO (6.7%), SLC26A7 (6.7%), and DUOX2 (3.3%).Monoallelic SLC26A4 mutations were found in two patients, one of them coexisting with two tandem biallelic deletions in SLC26A7. In 25 patients with thyroid dysgenesis, biallelicmutations in TSHRwere found in six patients (24%). Biallelic mutations in TSHB, PAX 8, NKX2-1, or HOXB3 were found once in four different patients. A monoallelic CDCA8 mutation was found in one patient. Mostmutations were novel, including three TG, two TSHR, and one each inDUOX2, TPO, SLC26A7, TSHB, NKX2-1, PAX8, CDCA8, andHOXB3. SLC26A7 andHOXB3 were novel genes associated with thyroid dyshormonogenesis and dysgenesis, respectively. Conclusions: TG and TSHR mutations are the most common genetic defects in Saudi patients with CH. The prevalence of other disease-causing mutations is low, reflecting the consanguineous nature of the population. SLC26A7 mutations appear to be associated with thyroid dyshormonogenesis.
