Optimized radioiodine therapy of Graves' disease: analysis of the delivered dose and of other possible factors affecting outcome

The best approach to radioiodine dose selection in the treatment of Graves' hyperthyroidism remains highly controversial. The formula to calculate the individual dose of I-131 to be delivered has been used for half a century and takes into account the thyroid mass, the effective half-life and the maximum uptake of I-131. The objective of the present study was to evaluate the accuracy of this formula by determining the relationship between the administered dose of I-131 calculated to deliver a target dose of 50 Gy to the thyroid and the actual exact organ dose. We further analyzed if therapeutic success, defined by euthyroidism following the individually calculated dose, can be predicted by different pretreatment parameters and particularly by organ dose. One hundred patients with a first episode of Graves' disease and who had received optimal thyroid irradiation after precise dosimetry were retrospectively reviewed. The patients were categorized according to their thyroid function (plasma free thyroxine (T-4) serum concentration) as eu-, hyper or hypothyroid during and 1 year after treatment. The relationship between the administered dose and organ dose was assessed by simple regression. We compared free T-4, free tri-iodothyronine, thyroid weight, the number of patients with antithyroperoxidase antibodies and TSH receptor autoantibodies. 24h urinary iodine excretion, I-131 uptake. and the exact dose of I-131 delivered to the thyroid as pretreatment variables, Although we found a correlation between administered dose (mCi) and organ dose (Gy) (r = 0.3, P = 0.003), the mean coefficient of variation for organ dose was 45%, Individualized radioiodine therapy enabled euthyroidism in 26% of patients and failed in 74% of patients (33% had persistent or recurrent hyperthyroidism and 41%; permanent hypothyroidism). I-131 uptake was significantly higher in the hyperthyroidism group in comparison with the euthyroid group. However, organ dose and other pretreatment variables did not differ among the three groups. In conclusion, these results confirm the low performance of individual dosimetry using what are established ratios, since the delivered dose to the gland, although correlated to the intended dose, is highly variable. The finding that other usual pretreatment variables are not different between groups, gives little hope for improving the way of calculating the ideal dose of radioiodine, We suggest to those not yet ready to give a standard or an ablative dose for Graves' hyperthyroidism that they abandon this way to calculate the I-131 dose.