[Determination of the serum concentration of radioiodine with 123I].

Though the half-life of 123I is much shorter than that of 131I, the determination of the radioiodine concentration 48 hrs after tracer administration (123I)48h can be done with the same precision. This precision is achieved by the fivefold dose given to the patient and the fourfold specific count rate. In 36 euthyroid patients with thyroidal autonomy the 123I concentration in serum was determined 48 hrs after tracer administration and was compared with the values obtained in a control group (n = 74). The diagnosis of thyroidal autonomy can be supported by the measurement of (123I)48h. The combination of (123I)48h and thyroidal radioiodine uptake seems to make the diagnostic decision more precise.