[Nucl Med Rev Cent East Eur.] BACKGROUND: The aim of the study was to analyze the effectiveness of radioactive 131I in hyperthyroid patients with confirmed lowered iodine uptake as compared to patients with an uptake of over 30%. MATERIAL AND METHODS: We retrospectively analyzed 53 consecutive patients aged from 29 to 84 (mean age 60 years) suffering from hyperthyroidism caused by Graves' disease or toxic nodular goitre. The patients were divided into 2 sub-groups: the 1st with a maximum iodine uptake of 18.7 +/- 3.2% (range, 11-23%) - 24 patients; the 2nd with a maximum iodine uptake of 27.1 +/- 2.1% (range, 24-30%) - 29 patients. The control group consisted of 50 patients treated with 131I with an iodine uptake of over 30%. Each patient was evaluated before, and 6 months after, treatment for fT3, fT4 and TSH with ECLIA; TRAb with RIA; ultrasound with a 7.5 MHz linear probe. The volume of the thyroid gland was determined using the Gutekunst method. All these factors underwent statistical analysis and were considered along with the results of clinical examinations. RESULTS: Clinical remission of hyperthyroidism was evident in 79.3% of both sub-groups, in total (83.3% and 75.3%, respectively). TSH was normalized in 62.3% of these patients (54.2% and 69.0%, respectively). The mean range of TSH levels increased from 0.081 microU/ml to 4.0 microU/ml after therapy; that is, from 0.087 microU/ml to 4.97 microU/ml in the 1st sub-group and from 0.076 microU/ml to 3.3 microU/ml in the 2nd sub-group. The volume of the thyroid gland was uniformly significantly lower, with a mean range of 40.5 ml before treatment and 21.7 ml afterwards. The results seen in both sub-groups were similar; only age and dose of radioiodine were slightly higher in the 1st, while mean uptake was higher in the 2nd. By comparison of these results to those of the control patients, we observed that the values of TSH, as well as thyroid volume and evidence of clinical remission, reflected those found in the control group. The mean dose of 131I was lower in the control group, that is 11.3 m Ci, as compared to the sub-groups as a whole, specifically, 15.7 mCi. The mean age of patients in the control group was slightly less than that of the study group (50.8 and 60, respectively). CONCLUSIONS: 1. The results of the treatment of patients with a low iodine uptake are similar to the results obtained in the group of patients with iodine uptake above 30% and therefore low iodine uptake should not be a contraindication for isotope I-131 therapy. 2. Additionally, we have demonstrated that a statistically significant decrease in thyroid volume is observed in all patients after the iodine isotope treatment which indirectly proves the effectiveness of the prescribed treatment, and that low thyroid iodine uptake is more frequently observed in elderly patients and in patients treated with iodine or anti-thyroid drugs.

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