Radiative Correction Effects of a Very Heavy Top
  If the top is very heavy, m_t >> M_Z, the dominant radiative correction effects in all electroweak precision tests can be exactly characterized in terms of two quantities, the rho-parameter and the GIM violating Z -> b bbar coupling. These quantities can be computed using the Standard Model Lagrangian with vanishing gauge couplings. This is done here up to two loops for arbitrary values of the Higgs mass. 
Riccardo Barbieri, Matteo Beccaria, Paolo Ciafaloni, Giuseppe Curci   and Andrea Vicere'