In recent years, the number of disseminated malignant tumors of the thyroid gland is growing rapidly, most often with medium or low histological differentiation, which worsens prognosis for patients’ recovery. The most important prognostic factors are the degree of primary tumor dissemination and morphological differentiation of the tumor. Taking into account the great impact of recurrences or prolongation of the disease on the prognosis of treatment, the authors investigated these indicators, and separately the survival in cases when they occur, depending on the degree of tumors’ dissemination and morphological differentiation. Objective — to study the rates of recurrence or prolongation of the disease, as well as their effects on the prognosis of treatment in patients with malignant thyroid tumors, depending on the spread of the primary tumor and the degree of its morphological differentiation. Materials and methods. The study involved 2.300 patients with malignant thyroid tumors after treatment, the rates of recurrence (prolongation) of the disease and survival depending on the occurrence of recurrence (prolongation) were investigated in comparison with the general sample in groups according to the degree of spread of the primary tumor T1—3a, T3b, T4a, and T4b and the degree of morphological differentiation of tumors (high, medium and low). Results. The increase in the percentage of relapses or prolongations of the disease in patients with disseminated thyroid malignant tumors after complex treatment depended on the degree of spread of the primary tumor and the degree of morphological differentiation of the tumor, which significantly worsened the prognosis for patient survival. The difference in survival rates among patients with relapse or prolongation of the disease in comparison with the general group (sample) increased with an increase in the spread of the primary tumor and a decrease in the degree of morphological differentiation of the tumors. Mortality rates at disease recurrences, regardless of their treatment, gradually increased, both with an increase in the spread of the primary tumor, and with a decrease in the degree of morphological differentiation of the tumor, and the worst indicator (0%) of survival was in the T4b group with tumors of low differentiation. Conclusions. The onset of relapse (prolongation) of the disease is a factor that significantly worsens the prognosis of patients, especially with widespread primary tumors of medium and low morphological differentiation. The main factors are the degree of spread of the primary tumor and the degree of morphological differentiation of the tumor.