  We use relativistic invariance to investigate two aspects of integrable
AdS${}_3$ string theory. Firstly, we write down the all-loop TBA equations for
the massless sector of the theory with R-R flux, using the recently discovered
hidden relativistic symmetry. Secondly, for the low-energy relativistic limit
of the theory with NS-NS flux we write down the S matrix, dressing factors and
TBA. We find that the integrable system coincides with a restriction to
AdS${}_3$ of the relativistic q-deformed AdS${}_5$ theory. We also comment on
the relativistic limit of the the small-$k$ NS-NS theory.
