  Seesaw mechanisms are the simplest and the most elegant way of generating
small masses for the active neutrinos $(m_\nu)$. In these mechanisms $m_\nu$ is
inversely proportional to the lepton number breaking scale $(M)$ that, in the
particular case of the type I seesaw mechanism, is the Majorana mass of the
right-handed neutrinos. In the canonical case right-handed neutrinos are
supposed to be heavy belonging to the GUT scale. With the advent of the LHC
people began to suppose these neutrinos having mass at TeV scale. In this case
very tiny Yukawa couplings are required. As far as we know there are no
constraints on the energy scales associated to the seesaw mechanisms. In what
concern 3-3-1 models, when we trigger the type I seesaw mechanism the lepton
number breaking scale that suppresses active neutrino masses contributes to the
masses of the standard gauge bosons. Current data on $m_W$ demands the
mechanism to be performed at GeV scale. As main implication we may have right
handed neutrinos with mass varying from few keVs up to hundreds of GeVs. We
also investigate the viability of the mechanism and found as interesting result
that in the case in which the right-handed neutrino masses belong to the range
keV-MeV scale, viability of the mechanism demands that the lightest of the
right-handed neutrinos be stable, which makes of it a natural dark matter
candidate, and that the lightest of the active neutrinos be much lighter than
the other two active neutrinos.
