The Helmholtz free-energy functional of the correlation functions for quantum spin-1/2 systems is studied, using field theoretical methods. It is shown that it does not coincide with the first Legendre transform of the effective action obtained by a Hubbard-Stratonovich transformation. It is instead related to the second Legendre transform of the original action, thereby confirming that approximations for this functional are conservative in the Baym and Kadanoff sense. The critical behaviour of the Ising model in a transverse field is discussed as an example.