  The dominant electroweak corrections to the production cross sections and
partial decay widths of a light Standard-Model Higgs boson, with mass M_H <<
2m_t, are due to top-quark loops. By means of a low-energy theorem, we study at
the two-loop level the leading shifts in the Higgs-boson couplings to pairs of
light fermions and gauge bosons induced by a sequential isodoublet of high-mass
fermions. For tree-level and loop-induced Higgs-boson couplings, these
corrections are of relative orders O(G_mu^2 m_F^4) and O(G_mu m_F^2),
respectively, where m_F represents a generic heavy-fermion mass, with m_F >>
M_W, M_H. Except for the Hbb-bar coupling, all results carry over to the case
of the top-quark-induced corrections. We discuss possible phenomenological
consequences of our results.
