The purpose of the above prospective study, which is based on the model of hyperthyroidism resulting from the solitary toxic adenoma of the thyroid gland, was to verify the hypothesis that application of radioiodine results ina necrosis of thyroid cells, in washing out of the previously formed hormones as well as in the growth of FT4 and FT3 levels in the serum thereby aggravating the clinical status of patients and possibly posing a danger to develop a thyrotoxic crisis. There supposedly exists a positive correlation between the received dose of radiation and the changes of FT4 and FT3. My study involved 30 patients, 21 of them female and 9 males, aged between twenty-seven and seventy-eight (54.1 14.8), all of them having received a therapeutic dose of 131I between January 15 and July 15, 1997 at the Clinic for Nuclear Medicine of the Ljubljana Clinical Center. All patients had a solitary autonomous nodule as well as a completely blocked accumulation in theremaining part of the gland in the scintigram with 99mTC, as well as a latent or manifest hyperthyroidism. All patients were treated with 925 MBq 131I. In order to calculate the received dose of radiation I applied my own method using gamma camera and computer. To mark the necrosis of thyroid cells,I used Tg and IL-6. During the follow-up of all the patients, I measuredthe serum concentrations of TSH, FT4, FT3, Tg and IL-6 prior to application as well as 12 hours, 1,2,3,5 and 12 days as well as three months following it. The average received dose of 131I was 247.81+-115.48 (102.5- 639.8) Gy. Within the first five days following the application of radioiodinethere were no statistically significant changes of FT4 and FT3 whereas there came to a notable statistically significant lowering of both FT4and FT3 (p