  Evaluating a lattice path integral in terms of spectral data and matrix
elements pertaining to a suitably defined quantum transfer matrix, we derive
form-factor series expansions for the dynamical two-point functions of
arbitrary local operators in fundamental Yang-Baxter integrable lattice models
at finite temperature. The summands in the series are parameterised by
solutions of the Bethe Ansatz equations associated with the eigenvalue problem
of the quantum transfer matrix. We elaborate on the example of the XXZ chain
for which the solutions of the Bethe Ansatz equations are sufficiently well
understood in certain limiting cases. We work out in detail the case of the
spin-zero operators in the antiferromagnetic massive regime at zero
temperature. In this case the thermal form-factor series turn into series of
multiple integrals with fully explicit integrands. These integrands factorize
into an operator-dependent part, determined by the so-called Fermionic basis,
and a part which we call the universal weight as it is the same for all
spin-zero operators. The universal weight can be inferred from our previous
work. The operator-dependent part is rather simple for the most interesting
short-range operators. It is determined by two functions $\rho$ and $\omega$
for which we obtain explicit expressions in the considered case. As an
application we rederive the known explicit form-factor series for the two-point
function of the magnetization operator and obtain analogous expressions for the
magnetic current and the energy operators.
