Administered dose (MBq) = Desired dose (MBq/g) x Gland weight x 100
Radioiodine uptake at 24 h (%)
A typical dose of 131I is in the range of 3-5 MBq/g, which for an average size gland and uptake, results in a dose of 250-400 MBq. Patients treated at the lower end of the dose range will have a 10-15% recurrence rate. The circumstances in which the dose should be increased include rapid thyroidal transit, recurrence of hyperthyroidism following previous therapy, large goitres and a history of recent treatment with anti-thyroid drugs.
Considering the substantial errors that are inherent in the inputs to the dose calculation, some centers prefer to treat with a standard dose according to whether the gland is mildly, moderately or greatly enlarged. For purposes of quality management it is helpful if all practitioners at a center can agree on the approach to be taken to dosing.
Multinodular glands should be treated at the higher end of the recommended dose range. Usually the uptake is somewhat lower than is the case with Graves' disease. Based on considerations of uptake, gland size and heterogeneity of uptake, a typical treatment dose will roughly fall between 1 and 4 GBq. A low iodine diet for 1-2 weeks before treatment will often allow a modest dose reduction (Table 3).
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