We consider relatively heavy neutrinos νH, mostly contributing to a sterile state νs, with mass in the range 10 MeV ≲ms≲mπ∼135 MeV, which are thermally produced in the early universe in collisional processes involving active neutrinos, and freezing out after the QCD phase transition. If these neutrinos decay after the active neutrino decoupling, they generate extra neutrino radiation, but also contribute to entropy production. Thus, they alter the value of the effective number of neutrino species Neff as for instance measured by the cosmic microwave background (CMB), as well as affect primordial nucleosynthesis (BBN), notably 4He production. We provide a detailed account of the solution of the relevant Boltzmann equations. We also identify the parameter space allowed by current Planck satellite data and forecast the parameter space probed by future Stage-4 ground-based CMB observations, expected to match or surpass BBN sensitivity.