Abstract Background Personalized molecular radiotherapy based on theragnostics requires accurate quantification of the amount of radiopharmaceutical activity administered to patients both in diagnostic and therapeutic applications. This international multi-center study aims to investigate the clinical measurement accuracy of radionuclide calibrators for 7 radionuclides used in theragnostics: ^99mTc, ¹¹¹In, ¹²³I, ¹²⁴I, ¹³¹I, ¹⁷⁷Lu, and ⁹⁰Y. Methods In total, 32 radionuclide calibrators from 8 hospitals located in the Netherlands, Belgium, and Germany were tested. For each radionuclide, a set of four samples comprising two clinical containers (10-mL glass vial and 3-mL syringe) with two filling volumes were measured. The reference value of each sample was determined by two certified radioactivity calibration centers (SCK CEN and JRC) using two secondary standard ionization chambers. The deviation in measured activity with respect to the reference value was determined for each radionuclide and each measurement geometry. In addition, the combined systematic deviation of activity measurements in a theragnostic setting was evaluated for 5 clinically relevant theragnostic pairs: ¹³¹I/¹²³I, ¹³¹I/¹²⁴I, ¹⁷⁷Lu/¹¹¹In, ⁹⁰Y/^99mTc, and ⁹⁰Y/¹¹¹In. Results For ^99mTc, ¹³¹I, and ¹⁷⁷Lu, a small minority of measurements were not within ± 5% range from the reference activity (percentage of measurements not within range: ^99mTc, 6%; ¹³¹I, 14%; ¹⁷⁷Lu, 24%) and almost none were outside ± 10% range. However, for ¹¹¹In, ¹²³I, ¹²⁴I, and ⁹⁰Y, more than half of all measurements were not accurate within ± 5% range (¹¹¹In, 51%; ¹²³I, 83%; ¹²⁴I, 63%; ⁹⁰Y, 61%) and not all were within ± 10% margin (¹¹¹In, 22%; ¹²³I, 35%; ¹²⁴I, 15%; ⁹⁰Y, 25%). A large variability in measurement accuracy was observed between radionuclide calibrator systems, type of sample container (vial vs syringe), and source-geometry calibration/correction settings used. Consequently, we observed large combined deviations (percentage deviation > ± 10%) for the investigated theragnostic pairs, in particular for ⁹⁰Y/¹¹¹In, ¹³¹I/¹²³I, and ⁹⁰Y/^99mTc. Conclusions Our study shows that substantial over- or underestimation of therapeutic patient doses is likely to occur in a theragnostic setting due to errors in the assessment of radioactivity with radionuclide calibrators. These findings underline the importance of thorough validation of radionuclide calibrator systems for each clinically relevant radionuclide and sample geometry.