Radioiodine therapy of autonomous thyroid tissue. Results with respect to pretherapeutic scintigraphic pattern and early response of triiodothyroninelevels

Aim: The goal of this retrospective study was to evaluate the results of ra dioiodine therapy of thyroid autonomy with respect to the underlying scinti graphic pattern, administered 1-131 doses and pretherapeutic Tc-99m and 1-1 31 uptake. Furthermore, early post-therapeutic free triiodothyronine (FT3) levels were measured during the first 6 weeks after radioiodine therapy wit h respect to a FT3-increase. Patients and Methods: Thyroid autonomy in 632 patients was followed for at least 3 months (mean 10.9 months). The patients were divided into several g roups based on scintigraph pattern of unifocal (UA), a multifocal (MFA), a focal/disseminated (FDA) and a disseminated (DA) autonomy. The early effect s of therapy on thyroid function were assessed by evaluation of FT3 values of 786 patients during the 1st, 2nd, 3rd, 4th and 6th week after therapy. Results: Successfully elimination of thyroid autonomy as defined by normali zation of FT3 and TSH levels was observed in 92.1% (582/632) of the patient s. In 2.2% (14/632) of the patients, immunogenic hyperthyroidism following 1-131 therapy was observed. There was no significant difference in the succ ess rates in patients with UA and MFA of 94.9% and 96.1%, respectively, usi ng similar target doses based on the amount of autonomous tissue. The succe ss rate in MFA (96.6%) was similar if target dose was calculated based on t he volume of the whole thyroid gland. Compared to MFA (96.6%), however, FDA and DA were associated with significantly lower success rate with 82.2% an d 75.5%, respectively, although the target doses were not significantly dif ferent. Considering the quantitative Tc-99m uptake prior to therapy, there was no significant difference in the success rate for Tc-99m less than or e qual to 3% and > 3%, while the success rate in patients with 1-131 uptake o f less than or equal to 50% was significantly higher (p = 0.032) than in we eks after radioiodine therapy revealed a dependence of the FT3 levels durin g the first 6 weeks after radioiodine therapy revealed a dependence of the FT3 decrease and FT3 increase on the scintigraphic pattern and thyroid func tion. Patients with FDA and DA with hyperthyroidism showed an increase of F T3 (> 2 to 14.7 pmol/l) 11 to 18% of the cases, during the first 2 weeks af ter therapy which occured significantly more frequently in patients with FD A and DA than in UA and MFA. A similar increase in FT3 level following radi oiodine therapy was observed significantly more often and earlier in patien ts with UA and MFA with hyperthyroid values than in patients with euthyroid FT3 levels, the decrease in FT3 level was observed significantly more freq uently in patients with UA and MFA only after 6 weeks following radioiodine therapy. Conclusion: In contrast to the multifocal autonomy (MFA), the target dose o f 150 to 200 Gy based on total thyroid volume did not result in a comparabl y high success rate of approximately 95% in disseminated and focal/dissemin ated types of thyroid automony. Therefore, an increase of target dose of 20 0 to 300 Gy is recommended. The transient FT3 increase particularly observe d in FDA and DA in the first weeks following radioiodine therapy makes shor t-term controls of thyroid function necessary, especially in patients with cardiac risk, in order to initiate necessary therapy.