  We perform a detailed analysis of the one-loop corrections to the light
neutrino mass matrix within low scale type I seesaw extensions of the Standard
Model and their implications in experimental searches for neutrinoless double
beta decay. We show that a sizable contribution to the effective Majorana
neutrino mass from the exchange of heavy Majorana neutrinos is always possible,
provided one requires a fine-tuned cancellation between the tree-level and
one-loop contribution to the light neutrino masses. We quantify the level of
fine-tuning as a function of the seesaw parameters and introduce a
generalisation of the Casas-Ibarra parametrization of the neutrino Yukawa
matrix, which easily allows to include the one-loop corrections to the light
neutrino masses.
