We present an extension of chiral perturbation theory that explicitly includes an isos- inglet scalar in the Lagrangian. The dynamical effects from the scalar state is of phe- nomenological relevance in theories where the mass of the isosinglet scalar is compara- ble to the mass of the pseudo-Goldstone bosons. This near-degeneracy of states is for example observed in certain near-conformal BSM models. From the Lagrangian we cal- culate the one-loop radiative corrections to the pion mass, the pion decay constant, and the scalar mass. We then proceed and use the results to fit numerical lattice data for an SU(3) gauge theory with $N_f=8$ light flavours in the fundamental representation.