This paper is concerned with flexural vibrations of composite plates, where piezoelastic layers are used to generate distributed actuation or to perform distributed sensing of strains in the plate. Special emphasis is given to the coupling between mechanical, electrical and thermal fields due to the direct piezoelectric effect and pyroelectric effect. A Reissner-Mindlin type single-layer plate theory is derived, and it is shown that coupling can be taken into account by means of effective stiffness parameters and effective thermal loading.