In this study we present the molecular basis of a total iodide organification defect causing severe congenital hypothyroidism. In the thyroid gland of the patient, thyroid peroxidase (TPO) activity and the iodination degree of thyroglobulin were below detection limits, and no TPO messenger ribonucleic acid was detectable by Northern blot analysis. Denaturing gradient gel electrophoretic analysis of the TPO gene of the patient revealed a homozygous mutation in exon 2. Sequence analysis showed the presence of a 20-basepair duplication, 47 basepairs down-stream of the ATG start codon. This duplication generates a frame shift, resulting in a termination signal in exon 3, compatible with the complete absence of TPO. Both parents of the patient are heterozygous for the same duplication, confirming the recessive mode of inheritance of the mutation.