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 he 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 anteriorly. Measured values were 10.144+/-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+/-1.2 days and 18.4+/-1.1 days.