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)IU(max)) needed for the calculation is usually determined with the use of (131)I. The purpose of the paper was to estimate (131)IU(max) on the basis of technetium uptake in the thyroid at 20 min ((99m)TcU(20min)). Eighty patients suffering from Graves’ disease were qualified for radioiodine therapy with measurement of fT(4), fT(3), thyroid-stimulating hormone and its receptor (TRAb). Prior to the treatment, all the patients were euthyroid. (131)IU(max) for each patient was determined according to the levels of (131)I after 24 h ((131)IU(24h)), while effective half-life (T (eff)) according to the measurements of (131)IU(24h) and (131)I uptake after 48 h ((131)IU(48h)). Additionally, on the day before measuring (131)IU(24h), (99m)TcU(20min) 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)IU(24h), TRAb and (99m)TcU(20min), and (99m)TcU(20min) and (131)IU(24h). The interdependence between (131)IU(24h) and (99m)TcU(20min) at the level of significance p < 0.05 is described by the following algorithms: (131)IU(24h) = 17.72 × ln ((99m)TcU(20min)) + 30.485, if TRAb < 10 IU/ml, and (131)IU(24h) = 18.03 × ln ((99m)TcU(20min)) + 38.726, if TRAb > 10 IU/ml. It is possible to predict thyroid iodine uptake (131)IU(24h) in Graves’ disease on the basis of measuring the uptake of (99m)TcU(20min). This shortens the time necessary for diagnosis and enables the calculation of (131)I activity using Marinelli’s formula.