We address the problem of reconstructing phase-shifting objects from their single-shot in-line holograms. We show that a phase-shifting object cannot be reliably recovered from its in-line hologram by noniterative reconstruction routines and that an iterative reconstruction should be applied. We demonstrate examples of simulated in-line holograms of objects with the following properties: purely absorbing, absorbing and phase shifting, and strong phase-shifting. We investigate the effects of noise and interference contrast in holograms on the reconstruction results and discuss details of an optimal iterative procedure to quantitatively recover the correct absorbing and phase-shifting properties of the object. We also review previously published reconstructions of experimental holograms and summarize the optimal parameters for retrieval of phase-shifting objects from their in-line holograms.