[Estimation of Effective Half Life of 131I in Radioiodine Therapy for Graves' Disease Using a Single Radioiodine Uptake Measurement]

Calculation of the absorbed radioiodine dose in the thyroid gland is a very important factor in 131I therapy for Graves' disease. In 131I therapy, the measurement of the effective half life (EHL) of radioiodine in the thyroid gland is indispensable for calculating the absorbed radioiodine dose. In some cases, however, EHL cannot be calculated because repeated measurements of thyroid uptake cannot be done. So, the authors developed a new method to estimate EHL using a single radioiodine uptake measurement. The activity of intrathyroidal 131I (thyroid uptake) and that of the surrounding area of the shielded thyroid (body background) were measured at 24 hours after the administration of 131I. The ratio of the value of body background to thyroid uptake was calculated. This ratio was multiplied by 100 and the calculated value was named the EHL-index. The relationship between actual measured EHL and the EHL-index was expressed by a regression line and/or a hyperbola. For the purpose of obtaining more actual values of EHL using the EHL-index, the new combined curve was composed of the regression line and the hyperbola. As for the value of calculated EHL compared with that of actual measured EHL, about 80 percent of calculated EHL values were within acceptable error ranges. These results indicate that EHL can be estimated by a single measurement 24 hours after administration of 131I. This method should be very useful in the therapy of Graves' disease with 131I when the thyroid measurement is performed only one time.