Therapeutic activity of radioiodine in Graves’ disease

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

Abstract

The therapeutic activity of ¹³¹I administered to patients with Graves’ disease can be calculated by means of Marinelli’s formula. The thyroidal iodine uptake (¹³¹IU_max) needed for the calculation is usually determined with the use of ¹³¹I. The purpose of the paper was to estimate ¹³¹IU_max on the basis of technetium uptake in the thyroid at 20 min (^99mTcU_20min). 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. ¹³¹IU_max for each patient was determined according to the levels of ¹³¹I after 24 h (¹³¹IU_24h), while effective half-life (T
_eff) according to the measurements of ¹³¹IU_24h and ¹³¹I uptake after 48 h (¹³¹IU_48h). Additionally, on the day before measuring ¹³¹IU_24h, ^99mTcU_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 ¹³¹IU_24h, TRAb and ^99mTcU_20min, and ^99mTcU_20min and ¹³¹IU_24h. The interdependence between ¹³¹IU_24h and ^99mTcU_20min at the level of significance p < 0.05 is described by the following algorithms: ¹³¹IU_24h = 17.72 × ln (^99mTcU_20min) + 30.485, if TRAb < 10 IU/ml, and ¹³¹IU_24h = 18.03 × ln (^99mTcU_20min) + 38.726, if TRAb > 10 IU/ml. It is possible to predict thyroid iodine uptake ¹³¹IU_24h in Graves’ disease on the basis of measuring the uptake of ^99mTcU_20min. This shortens the time necessary for diagnosis and enables the calculation of ¹³¹I activity using Marinelli’s formula.



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