Measurement of serum TSH level by ultrasensitive method in inhabitants of endemic goiter area supplied with iodized salt for 25 years.

Recently, we surveyed thyroid function and TSH concentration of villagers in an endemic goiter area where iodized salt had been supplied for 25 years; it was found that the serum FT3 and TSH levels determined with immunoradiometric analysis (IRMA) were higher and the FT4 level was lower than that of the controls. It was shown that the inhabitants of the endemic goiter area had subclinical hypothyroidism based on the "ultrasensitive" method for TSH assay. We suggest that the best biochemical techniques for monitoring the iodized salt prophylaxis program and the physiological response of the villagers to iodine should be the periodical measurement of serum TSH level using ultrasensitive assay and determination of FT4 level.