Deposition Fraction and Effective Half Life of Iodine-131 in Human Thyroid

The contents of 131I in the thyroid of radioisotope workers were measured with a large NaI (T1) detector. For this purpose, a practical and convenient method was developed to estimate the fraction of 131I in the thyroid of that in the total body, f2, as well as the contents of 131I in the thyroid.The observed value of 131I content in the thyroid varied to some extent with the difference of the relative position between the thyroid and the detector. Three per cent of the coefficient of variation. was observed at actual measurements for human subjects.The values of f2 for 63 people who inhaled 131I were determined. They fit a log-normal distribution on ranging widely from 0.17 to 0.68. The geometrical mean was calculated as 0.28. In addition, the change with time of the 131I contents in the thyroid was followed for 11 cases to determine the effective half-lives. These values ranged from 4.9 to 7.6 days and the mean value with the standard deviation was 6.6±0.7 days. This mean value is a little less than the value for the standard man (7.6 days) given by the ICRP.