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The paper discusses the calculation of the combined standard uncertainty of the absorbed dose measurements using the thermoluminescent dosimetry (TLD). This procedure serves as a key tool for independent dose verification in clinical practice, particularly within “dose-by-mail” audit programs. Ensuring proper metrological traceability and correct uncertainty determination are mandatory requirements for the accreditation of such dosimetry services. The calculation is performed in accordance with the GUM and IAEA international recommendations and involves determining the limits of the combined uncertainty. The combined standard uncertainty of the absorbed dose is calculated as the combined influence of the uncertainties of individual factors in the calculation formula, specifically: the calibration coefficient of the TL system, correction factors for fading, the non-linearity of the “dose-TL signal” dependence, and the presence of the detector holder. The most significant contribution to the combined uncertainty comes from the uncertainty of the calibration coefficient, which is associated with the accuracy of delivering the absorbed dose to water according to the ionization chamber readings. To minimize it, modern methods of statistical data processing, obtained experimentally, were applied. Experimental studies were conducted at various stages using a therapy unit and a water phantom, the state primary measurement standard of the absorbed dose, and a PMMA phantom in a horizontal beam geometry. Under all irradiation conditions, the combined standard uncertainty of the absorbed dose did not exceed ±3%, which complies with the IAEA requirements for TLD audit centres. The proposed approach provides a reliable basis for metrological support of dosimetry in clinical practice and can be implemented in the work of dosimetry audit services.