National diagnostic reference levels (DRLs) are an essential tool for optimizing protocols and protecting patients from an unjustified radiation exposure. The aim of this study was to conduct a nationwide survey of adult patient doses received from nuclear medicine procedures and update national DRL values for most common procedures. Data on the radioactivity administered to standard adult patients was collected from all Lithuanian hospitals, using in practice nuclear medicine applications, between 2017 and 2022. Twelve of the most commonly performed nuclear medicine diagnostic procedures: myocardial perfusion scintigraphy (two parts), thyroid scintigraphy, lung perfusion scintigraphy, bone scintigraphy, dynamic renal scintigraphy (divided into two procedures), parathyroid scintigraphy, hepatobiliary scintigraphy, lymphoscintigraphy, neuroendocrine tumor scan, oncology PET/CT imaging were included. The 75th percentile of dose the distribution was estimated for establishing national DRLs. During data collection period, the nuclear medicine imaging equipment were upgraded. The procedures were completed with 12 SPECT and SPECT/CT gamma cameras (two with CZT detectors), 2 cardiac-centred gamma cameras with CZT detectors and 2 PET/CT scanners. Data was collected on 7979 administered activities to the patients. For each procedure, depending on its frequency, 50 to 580 administered radiopharmaceutical activities were selected. Based on the collected data, national DRLs were updated in 2021 for three procedures. A large part of procedures were performed with scanners manufactured between 2008 and 2011. In the past few years, the main Lithuanian hospitals that provide nuclear medicine services have updated their SPECT/CT systems. In the future, the data collection process should be continued in order to reassess DRLs with respect to new scanning systems (e.g. with a CZT detectors and newer PET/CT technology). Variations in the prescribed radiopharmaceutical activities showed that optimization and harmonization procedures may be performed in some hospitals.