IMPROVED DOSE CONCEPT FOR RADIOIODINE THERAPY OF MULTIFOCAL AND DISSEMINATED FUNCTIONAL THYROID AUTONOMY

The present study analyzes the improvement of the outcome of radioiodi ne therapy in nonimmunogenic hyperthyroidism by adapting the target do se to the Tc-99m-pertechnetate thyroid uptake under suppression (TcTUs ) prior to radioiodine therapy. The TcTUs is a substitute for the non- suppressible iodine turnover. The 89 patients presented with a basal t hyrotropin level of <0.1 mU/1, normal values far Gee triiodothyronine and thyroxine and with multifocal or disseminated thyroid autonomy. Th ese terms describe the scintigraphic distribution pattern of autonomou s iodine turnover. Thirty-two patients had a TcTUs between 1.6 and 3.2 % (group A) and 57 had a TcTUs > 3.2% (group B). Fifty-Eve patients (t hree of group A and 52 of group B) were treated previously for overt h yperthyroidism with antithyroid drugs. Target doses of 150 and 200 Gy were used in both groups and 300 Gy in group B only. Six months after radioiodine therapy, a basal TSH level of greater than or equal to 0.5 mU/I as criterion of therapy success was observed in 94% of group A a nd in 54% of group B. Further differentiation of group B shows an incr easing success rate with the target dose used: 45% after 150 Gy, 50% a fter 200 Gy and 90% after 300 Gy. In patients with a basal TSH level o f <0.5 mU/1 after radioiodine therapy, the TcTUs was evaluated again. Persistence of functional thyroid autonomy, defined as TcTUs > 1.6%, w as found in 89% (one patient of group A, 24 patients of group B) and s till observed a high extent of autonomous function in 25% of them, evi denced by a TcTUs > 3.2% (seven patients of group B, target doses of 1 50 or 200 Gy). No case of overt hypothyroidism was observed within the first 6 months after radioiodine therapy and no difference was found in therapy outcome between multifocal and disseminated thyroid autonom y. As a consequence, the target dose should be adapted to the TcTUs pr ior to radioiodine therapy in the range of 150-300 Gy to the total thy roid gland.