Radio-iodine treatment (RaIT) is a well-established method for treatment of hyperthyroidism due to Graves’ disease (GD), toxic adenoma (TA), toxic multinodular goiter (TMG).
The optimal 131-Iodine (131I) activity to be administrated is still matter of debate.
We report our experience with calculated activity of 131I.
Sixty-nine (16 GD, 39 TA, 14 TMG) patients [44 females and 25 men; mean age 64.2±12.6 years] were enrolled. All but 17 patients (24.6%) were overtly hyperthyroid and 41 (59.4%) had received anti-thyroid drugs, withdrawn 5-10 days before RaIT. Twenty patients were anti-thyroid antibody positive (16 with GD). All patients underwent: a) Thyroid Scintigraphy (TS) and Radioiodine Thyroid Uptake (RTU) measurements from 3 to 168 hours after 131I administration (1.8 MBq); b) thyroid ultrasonography (TU), to calculate the volume of the “hot” thyroid nodule(s) or of the whole gland. In TMG and TA patients, the “effective” volume of the hot nodule(s) was calculated subtracting the volume of involution area(s).
Mean RTU was 49.1±13.8%. Therapeutic activity was calculated by Snyder formula modified according to the “effective” volume. In 11 patients with TMG the activity was calculated for all the “hot” nodules. Mean administrated activity was 303±135.4 MBq and mean adsorbed dose was 223±48.6 Gy. A woman with overt hyperthyroidism due to a 50 mm-TA was hospitalized and treated with 934 MBq of 131I. Mean follow-up was 47.4±16.9 months.
Fifty patients became euthyroid within a mean of 3.2±2 months after RaIT. Fourteen patients (10 with GD) developed overt hypothyroidism within a mean of 5.6±5.0 months. Two women (TMG and GD) developed sub-clinical hypothyroidism respectively 12 and 16 months after RaIT. Other 3 with GD showed recurrence of hyperthyroidism after 2 months and underwent a second RaIT (with standard activity). The patient treated with 934 MBq of 131I developed hypothyroidism 3 months after RaIT.
TS was performed in 34 TMG or TA patients about 3 months after RaIT, demonstrating partial (22 patients) or total (12 patients) ablation of the “hot” area(s). In 15 patients with TA, the TU performed about 4 months after RaIT showed a significant reduction of the nodule volume (t= 2.43, p 0.03). No side effects were observed during follow-up.
Calculated activity of 131I permitted to obtain euthyroidism in 50/69 (72.5%) patients [in 48/53 (90.6%) patients considering only TA and TMG cases]. The cure-rate was higher (95.6%) if we consider hypothyroidism as a successful outcome. In a previous experience with standard activity of 131I we found a lower cure-rate (respectively 69.6% and 90.2%), despite a significantly higher administered activity (mean 541 MBq vs 303 MBq; p<0,05). It follows that calculated activity will correspondingly reduce both the dose absorbed by “critical organs” (stomach, bladder) and the radiation exposure to other people.

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