Influence of thyroid volume reduction on calculated dose in radioiodine therapy of Graves' hyperthyroidism

Administration of radioactive iodine (I-131) is an effective treatment for hyperthyroidism due to Graves' disease. Recently several investigators have shown that the success of this therapy may depend on the absorbed dose to the thyroid. Thyroid dose varies inversely with the mass of the gland. Much experimental evidence demonstrates that a reduction of the thyroid volume (mass) may occur after radioiodine therapy. In this work we evaluate the influence of the volume reduction on the calcu lation of the absorbed dose to the thyroid. A mathematical model of thyroid mass reduction after I-131 therapy is presented, based on masses evaluated with ultrasonography of ten patients treated in the endocrinology departme nt of our hospital. This model was applied to the general formula for calcu lation of the thyroid doses in these patients. The dose values obtained con sidering a reduction of thyroid mass after the treatment are often quite di fferent from those obtained without considering change in mass (from 9% to 30% greater). We conclude that the consideration of thyroid mass reduction is important for an accurate estimation of the calculated dose.