Dealing with thermomechanical analysis of structures this paper presents a finite element formulation for the nonlinear analysis (in von Kármán sense) of shear deformable stiffened and unstiffened flat panels subjected to machanical and linear through the thickness temperature fields loadings. The obtained governing nonlinear system of algebraic equations is solved by application of Newton-Raphson method accounting arc-length-type strategies. Consistent tangent stiffness matrix have been derived. The few numerical results displayed demonstrate both the accuracy and the potentiality of the proposed model. In particular, the effects of reinforcement geometrical parameters on the large deflection effects have been investigated. Coupling effects resulting from reinforcements, transverse pressure and through the thickness temperature gradient are analyzed. Furthermore, thin plate approximations are evaluated by comparing related results to transverse shear deformable plate analysis.