We encountered a Japanese patient with goitrous hypothyroidism due to iodide organification defect in the thyroid gland. Sequence analysis identified two novel mutations (E378K in exon 8 and a heterozygous 10 base deletion of the intron 15-exon 16 boundary) in the thyroid peroxidase (TPO) gene. As individuals with goitrous hypothyroidism caused by TPO gene mutation develop thyroid cancer, regular and careful follow-up for such patients must be done.