  The strong topological insulator in 3D is expected to realize a quantized
magneto-electric response, the so-called axion response. However, many of the
materials predicted to be topological insulators have turned out to be
metallic, with bulk Fermi surfaces. Following the result of Bergman et al.
(Phys. Rev. B 82, 195417 (2010)) that the helical surface states of the
topological insulator persist even when the band structure gap is closed, we
explore the fate of the magneto-electric response in such systems. We find a
non-quantized magneto-electric coupling remains once a bulk Fermi surface opens
- a non-universal axion response. More generally we find that higher
dimensional analogs of the intrinsic anomalous Hall effect appear for
\emph{every} Chern form - non-quantized response coefficients for gapless
systems, as opposed to quantized transport coefficients in gapped systems, both
with a topological origin. In particular, the non-quantized magneto-electric
response in 3D descends from the intrinsic anomalous Hall effect analog in 4D.
