Biological Half-life of Iodine in Adults with Intact Thyroid Function and in Athyreotic Persons

A joint project between the Human Monitoring Laboratory (HML) and the Ottawa Hospital has measured the retention of 131I in patients who have received the radioiodine diagnostically. Thirty-nine subjects with intact thyroid glands and nine athyreotic subjects were measured in the HML's whole-body/thyroid counter to determine the retention of 131I following its medical administration. The average biological half-life of 131I in 26 euthyroid subjects was found to be 66.1 ± 6.3 days which may be statistically significantly lower than the ICRP recommended value of 80 days. Nine hyperthyroid patients had a mean biological half-life of 38.2 ± 8.6 days and in three hypothyroid patients the corresponding value was 29.3 ± 8.8 days. Thyroid 131I uptake was measured in a conventional clinical fashion at the Ottawa Hospital Civic campus 24 h after oral administration of the radioiodine using a collimated thick sodium iodide detector placed over the neck arteriorly. Measured values were 0.144 ± 0.009, 0.314 ± 0.035 and 0.045 ± 0.010 of the administered dose in euthyroid, hyperthyroid and hypothyroid patients respectively. The euthyroid range at the hospital is 0.06 - 0.22. Uptake was significantly lower for the euthyroid group than the ICRP value of 0.3. The radioiodine retention in athyreotic subjects followed a two compartment model with biological half-lives of 1.0 ± 0.2 days and 18.4 ± 1.1. days.