Exhalation of I-131 after radioiodine therapy: Dependence on radioactivity applied

Abstract

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Objectives: Previous results suggested that in radioiodide-treatment (RIT), radioactive iodide must pass through the thyroid glands in order for it to be exhaled in a predominantly organically bound form. Our aim was to establish, through animal experiments, whether the extend and chemical form of the exhaled I-131 iodine is dependent on the dose of radioactivity applied. Methods: Five different doses of I-131 (0.1, 1, 2, 10 and 23 MBq/animal) were given orally to female mice (strain: Balb/c, weight 25 g), and the exhaled iodine was collected in an air flow sampler. Different forms of iodine (aerosol, elementary, organically bound and other forms) were adsorbed onto various filters over a 5-hour collecting period, according to their chemical forms. Levels of radioactivity in the filters were measured with a well counter, 5 mice being studied for each dose of radioactivity. Results: The absolute amount of exhaled radioactivity in Bq rose exponentially in relation to the dose of activity applied. The proportion of exhaled to applied activity at 1-23 MBq remained constant, at c. 0.3%. On administration of just 0.1 MBq, a markedly higher percentage (0.9%) was exhaled. The lowest fraction of iodine exhaled was that bound to aerosols, at all activities tested, while the highest fraction of the exhaled radioiodine was in an organically bound form. This portion lay between 67 and 78% at all activity doses administered. The ratios of the various forms of exhaled radioiodine to one another showed no significant dependence on activities applied. Conclusions: Increasing the activity applied has no effect on the ratios of the various chemical forms of the exhaled radioiodine to one another, but does produce a predictable increase in the total level of radioactivity exhaled. This conclusion can be drawn not only from the results obtained from the present study on mice but also from values obtained in previous studies on RIT patients. In mice, the ratio of exhaled activity to applied activity was about ten times higher than in patients, which could possibly be attributed to differences in the metabolism of mice and humans.