Calculation of therapeutic activity of radioiodine in Graves' disease by means of Marinelli's formula, using technetium ( 99m Tc) scintigraphy.

The therapeutic activity of 131 I administered to patients with Graves' disease can be calculated by means of Marinelli's formula. The thyroidal iodine uptake (131 IUmax ) needed for the calculation is usually determined with the use of 131 I. The purpose of the paper was to estimate 131 IUmax on the basis of technetium uptake in the thyroid at 20 min (99m TcU20min ). Eighty patients suffering from Graves' disease were qualified for radioiodine therapy with measurement of fT4 , fT3 , thyroid-stimulating hormone and its receptor (TRAb). Prior to the treatment, all the patients were euthyroid. 131 IUmax for each patient was determined according to the levels of 131 I after 24 h (131 IU24h ), while effective half-life (T eff ) according to the measurements of 131 IU24h and 131 I uptake after 48 h (131 IU48h ). Additionally, on the day before measuring 131 IU24h , 99m TcU20min was calculated for each patient. It was demonstrated that there existed a correlation, with statistical significance at p < 0.05, between the following pairs of values: TRAb and 131 IU24h , TRAb and 99m TcU20min , and 99m TcU20min and 131 IU24h . The interdependence between 131 IU24h and 99m TcU20min at the level of significance p < 0.05 is described by the following algorithms: 131 IU24h  = 17.72 × ln (99m TcU20min ) + 30.485, if TRAb < 10 IU/ml, and 131 IU24h  = 18.03 × ln (99m TcU20min ) + 38.726, if TRAb > 10 IU/ml. It is possible to predict thyroid iodine uptake 131 IU24h in Graves' disease on the basis of measuring the uptake of 99m TcU20min . This shortens the time necessary for diagnosis and enables the calculation of 131 I activity using Marinelli's formula.