Destabilization of human transthyretin (TTR) has been implicated in its misfolding and aggregation. A previous study on the neutron crystal structure of TTR suggested that a large hydrogen bond network around H88 which includes water molecules is significantly involved in the stability of wild-type TTR (WT-TTR). Here, we demonstrate that the H88R mutant associated with amyloid cardiomyopathy is substantially destabilized compared with WT-TTR. In order to clarify the role of H88 and the hydrogen bond network in the stability of TTR, we determined the thermodynamic stability and the crystal structure of H88 mutants (H88A, H88F, H88Y, and H88S). Our results suggest that in some cases TTR is destabilized due to alterations in bound water molecules as well as structural changes in TTR itself.