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Background: The prevalence of congenital hypothyroidism (CH) increased during the period 1994-2002 in Japan. The reasons for these recently described increases in the prevalence of CH remain unclear. Moreover, the proportion of patients with different etiologies CH in the more recently diagnosed patients has not been established. In this study, we determined the etiologies of CH that were detected by neonatal screening in Niigata refecture, Japan., Methods: A total of 100 patients having a diagnosis of CH (41 men and 59 women, aged 5-19 years old) were evaluated. To determine the etiology of CH, the patients underwent a ¹²³I thyroidal radioiodine uptake test, a scintigram, a saliva to plasma radioiodine ratio analysis, a perchlorate discharge test, thyroid ultrasonography, measurements of thyroidal function and thyroglobulin, and a thyrotropin (TSH)-releasing hormone tolerance test., Results: Patients with overt CH (n=34, elevated TSH levels with low free thyroxine levels) made up 34% of the total group, 56% of the patients had subclinical CH (n=56, elevated TSH levels with normal free thyroxinelevels), and 10% had normal thyroid function. These were patients who were considered to have transient hypothyroidism or hyperthyrotropinemia. Thyroid dysgenesis was the diagnosis in 73% of patients with overt CH, and the most of these had ectopic thyroid tissue. In contrast, thyroid dysgenesis was the diagnosis in only 36% of the patients with subclinical CH., Conclusions: Only 50% of our patients with CH detected by neonatal screening had thyroid dysgenesis. With an increase in the percentage of patients with subclinical hypothyroidism, the prevalence of thyroid dyshormogenesis has increased. Studies of the frequency and etiology of CH should consider overt and subclinical CH separately.

A total of 1,228,551 newborn babies, who were almost all of babies born in Osaka for 14 years (168 months), were screened for congenital primary hypothyroidism by an identical mass-screening program using the thyrotropin method, and 429 patients with hypothyroidism due to thyroid dysgenesis (dysgenetic hypothyroidism) were found. The occurrence of the patients in every month was not random but episodic and the incidence was higher in the late autumn (from October to December). These observations support a hypothesis that some environmental factors may cause this disorder overtime and the possibility of relation with intrauterine viral infection was discussed.

The seasonal distribution of birth dates of 31 patients with sporadic cretinism due to thyroid dysgenesis was analyzed in Osaka area for 14 years. The incidence was statistically high in the summer months. A hypothesis that some environmental factors such as viral infection may cause the disease is proposed.

The seasonal and chronological distributions of birth dates of 820 patients with congenital hypothyroidism due to thyroid dysgenesis were analysed in eight areas in the world. The incidence had some seasonal variations in certain areas. These observations suggest that some environmental factors cause this disease.