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Summary: the article discusses the reasons for the possible ineffectiveness of radioiodablation in patients with differentiated thyroid cancer when using adequate individually calculated activities of 131I. Based on a prospective analysis of post-therapeutic whole-body scintigraphy, laboratory parameters, dosimetric control and clinical manifestations after radionuclide therapy, significant criteria for the possible failure of standard combination therapy are identified and a change in treatment tactics in this group of patients is proposed.Purpose: search for the reasons for unsatisfactory results of radionuclide treatment in patients with differentiated thyroid cancer. Materials and Methods: analysis of 2260 patients with differentiated thyroid cancer who received combined treatment using traditional preparation methods using thyrotropin alpha, from which a group was identified with a high level of 131I. that was unexpected at the pre-therapeutic stage against the background of early radiation diseases and low dosimetry indicators. In patients, laboratory parameters of TSH, thyroglobulin and thyroglobulin antibodies were assessed using radioimmunoassay and clinical manifestations of radiation conditions, taking into account the prescribed therapy; The scintigraphy of the thyroid bed with 99mTc-pertechnetate, post-therapeutic scintigraphy with 131I upon exiting the «closed mode» and dosimetric control under standard conditions every 24 hours after taking the therapeutic activity were performed.Material and methods: prospective single-center analysis of data from 2260 patients with differentiated thyroid cancer who received combined treatment using the traditional preparation method and the use of thyrotropin alfa, from which a group with an unexpected high RP uptake at the pre-therapeutic stage was identified against the background of the absence of early radiation complications and low dosimetry indicators. In patients, laboratory parameters of TSH, TG and AT-TG were assessed using radioimmunoassay and clinical manifestations of radiation reactions, taking into account the prescribed therapy; scintigraphy of the thyroid bed with 99mТс-sodium pertechnetate, post-therapeutic scintigraphy with 131I upon exiting the «closed mode» and dosimetric monitoring with an assessment 48 hours after ingestion of therapeutic activity. Results: analyzed the data of a group of patients in whom there was a marked discrepancy between the usually interdependent results of scintigraphy before and after RIA, which subsequently changed the treatment tactics in their relation. A statistically significant difference was found between the selected groups in terms of the level of stimulated TSH (in patients with predictable (73,9±31,8 mU/L) and unexpected (114,1±30,5 mU/L) uptake of 131I (p=0,0218); by the values of the dose rate 48 hours after the intake of the calculated activity of 131I (p=0,000005). In addition, we additionally analyzed the effect of traditional and drug methods of stimulating thyrotropinemia during preparation for radionuclide ablation. Withdrawal of levothyroxine and with the use of rhTSH, TSH indices were 80,7±27 mU/L and 123,3±75,8 mU/L, respectively (p=0,035), and the levels of stimulated thyroglobulin were 10,2±4,0 ng / ml and 3,2±2,5 ng / ml (p=0,041). Conclusion: the obtained results show that the recommended empirical level of TSH > 30 mU/L is not synonymous with the activation of sodium iodine symporter and the viability of RIA, and also make it possible to distinguish a group of patients with delayed/ partial activation of the symporter. In the continuation of this study, we plan to change the protocol for preparation for radioiodablation and find criteria for pre-therapeutic diagnosis of this group of patients