PURPOSE: Thyroxine is the hormone indispensible to the growth and development of infants. We made this study to confirm the influence of temporary depression of serum thyroxine levels on the development of intelligence. METHODS: I adopted as the study group 14 patients diagnosed with depression at the pediatrics department of Pusan National University Hospital from April of 1991 to October of 1997, and selected as the control group 24 infants born with normal functioning of thyroid glands in the same period. From August 18 to September 3 2003, a clinical psychologist carried out Korean Wechsler Intelligence Scale for Children on the patient group and the control group. According to the guidelines, they conducted 12 small tests, gave them marks and expressed the intelligent quotients by the conversion grade in seven articles. RESULTS:The patient group had meaningfully lower grades than the control group in three(similarity, vocabulary, digit scan) out of six articles of the small test of verbal scales, and the former had meaningfully lower grades in two(picture completion, object assembly) out of six articles of the small test of performance scales by K-WISC-III. The I.Q. shows the group of patients was meaningfully depressed in six(verbal IQ, performance IQ, global IQ, verbal comprehension, perceptional organization, working memory) out of seven articles; out of the six articles, the working memory was most depressed. CONCLUSION: Transient hypothyroidism can bring the mental retardation of children, so we think that there should be further studies for appropriate thyroid hormone replacement therapy on these patients.