Results for Precision Observables in the Electroweak Standard Model at   Two-Loop Order and Beyond
  Higher-order contributions to the precision observables \Delta r, \sin^2\theta_eff and \Gamma_l are discussed. The Higgs-mass dependence of the observables is investigated at the two-loop level, and exact results are derived for the Higgs-dependent two-loop corrections associated with fermions. The top-quark corrections are compared with the results obtained by an expansion in the top-quark mass up to next-to-leading order. For the pure fermion-loop contributions to \Delta r results up to four-loop order are derived. They allow to investigate the validity of the commonly used resummation of the leading fermionic contributions to \Delta r. 
G. Weiglein