Introduction: One of the most important thyroid disorders is hyperthyroidism resulting from Graves’s disease. One of the most common therapies is using radioactive iodine, called iodine therapy. This study aims to identify the factors effective on TSH level variation in the patients with Graves’s disease under treatment with radioactive iodine.

Materials and Methods: This is a longitudinal-observational study. 201 patients with Graves’s disease, who were under treatment with radioactive iodine, were entered into the study. TSH levels in the patients’ blood were measured prior to the treatment with radioactive iodine, and 2, 4 and 6 months after the treatment. The marginal model by generalized estimating equations, and mixed effects model were fitted in order to analyze the effect of each predictive variable on the trend of TSH levels. The statistical analysis was performed by using R3.5.2 software, and for all the tests, the level of significance was considered 0.05.

Results: Mean age of patients was 37.54 ± 13.66 years, out of which 61.7% were female. In both the methods, the interaction of time duration, and baseline TSH were significant (P < 0.05). In addition, fitting the mixed effect model showed that the dose of Methimazole had significant effect on the TSH level (P < 0.05).

Conclusions: In this study, results of both the models were similar. The effects of baseline TSH, dose of administered Methimazole and the interaction of time and the dose of radioactive iodine led to the increase of TSH in the patient’s bloodstream.

hyperthyroidism, radioactive iodine, generalized estimating equations, mixed effects model.