Purpose <p>Internal dosimetry in nuclear medicine commonly assumes a fixed upright standing posture of subjects. The use of four recent therapeutic radionuclides with longer half-lives causes long exposure to patients during different daily activities and postures. This study aims to evaluate the effect of posture change on organ absorbed doses for seven radionuclides: diagnostic photon/positron-emitters (⁶⁴Cu, ⁶⁸Ga, ^99mTc) and therapeutic alpha/beta-emitters (¹⁷⁷Lu, ²¹¹At, ²¹²Pb, ²²⁵Ac).</p> Method <p>Whole body Japanese adult male and female digital human phantoms (National Institute of Information and Communications Technology, Japan) in standing and sitting postures were used in this study. Based on the reference biodistribution of ¹⁷⁷Lu-DOTATATE in kidneys, liver, spleen and remainder of the body, generated 28 time-integrated activity maps (seven radionuclides, two genders, two postures) were used for RT-PHITS Monte Carlo simulations to calculate organ absorbed doses. A comparative analysis of the organ absorbed doses between the two postures was specifically conducted for the liver, kidneys, and spleen.</p> Results <p>Large differences in organ absorbed doses between the two postures were observed for diagnostic radionuclides, with ⁹⁹ᵐTc exhibiting more than 15% kidney dose differences. In contrast, therapeutic radionuclides showed less than 4% differences, as their energy deposition is dominated by self-dose.</p> Conclusion <p>Incorporating posture into internal dosimetry may improve the accuracy of absorbed dose estimation and support more personalized applications for diagnostic nuclear medicine.</p>