Radioiodine therapy in Graves' hyperthyroidism: determination of individual optimum target dose

The aim of this study was to investigate the results of different volume-dependent target doses on clinical outcome 6 months after radioiodine therapy (RITh) and its correlation with post therapeutic thyroid volumes (Vpost) in patients with Graves' disease. This analysis was designed to determine factors improving the results of radioiodine therapy without increasing target doses generally, as has been recommended recently. We studied consecutive data from 102 patients with Graves' disease, who had initial radioiodine therapy between 1991 and 1995. The 131I activities were calculated according to the formula of Marinelli. In addition to the normal calculation individual target doses were adjusted to the thyroid volumes of each patient before therapy. For statistical evaluation, the patients were divided into three subgroups of comparable sample sizes: Group I included those with a thyroid volume <15 ml before therapy. Group II included those ranging from a 15-25 ml volume before therapy and group III included those with thyroid volumes >25 ml. Laboratory thyroid parameters and thyroid volumes were measured in those groups before and 6 months after therapy.

Analysis of all patients revealed a significantly higher rate of hypothyroidism (54%) and fewer cases of hyperthyroidism (15%) six months after therapy in cases with Vpost smaller than 8 ml. The median Vpost needed to achieve an optimum therapeutic success rate (rate of eu- or hypothyroidism) was smaller than 5 ml in group I and smaller than 10 ml in group II. Therapeutic success was associated with different target doses in each group, 150,220 and 260 Gy for groups I, II, and III respectively.

Post therapeutic thyroid volumes correlated significantly with clinical outcome six months after therapy. An adjustment of the target doses based on thyroid volumes before therapy will lead to an appropriate reduction of thyroid volumes. Thus, in the individual case clinical outcome could be improved without applying higher target doses in all patients. This would ensure a better utilization of limited resources in medical care e.g. through a shorter hospital stay.