The establishment of an in vivo internal monitoring programme requires the use of phantoms to represent an activity distribution of an incorporated radionuclide within the body. The aim of this study was to quantify the impact of the phantom geometry on the minimum detectable activity (MDA) of an incorporated radionuclide. The MDA was assessed for two instruments: a conventional radiation protection instrument and a portable gamma spectrometer. Four phantoms were considered: two physical phantoms, a simplified torso phantom and a commercial whole body phantom, as well as two numerical phantoms, the reference adult male and female voxel phantoms published by the International Commission on Radiological Protection (ICRP). The phantoms were loaded with activity at the level of the thorax and abdomen using reference sources of Co-57, Ba-133, Cs-137, Co-60 and Eu-152. The MDA for both instruments was experimentally assessed using the two physical phantoms. The experimental setup was modelled in GEANT4 and the simulated instrument responses were validated by the experimental data. The Monte Carlo model was then used to compute the instruments response and corresponding MDA when using the ICRP voxel phantoms. The simplified torso phantom provided one of the highest MDA estimates, up to a factor of 5 higher than the ones obtained with the voxel phantoms when considering a Co-57 source. Depending on the considered source distribution within the phantoms, physical phantoms may lead to an underestimation of the MDA when compared to more complex and anatomically accurate numerical phantoms. This work presents a quantitative comparison between the MDA obtained with different phantoms and radionuclide distributions.