  The theory of QED corrections to hyperfine structure in light hydrogenic
atoms and ions has recently advanced to the point that the uncertainty of these
corrections is much smaller than 1 part per million (ppm), while the
experiments are even more accurate. The difference of the experimental results
and the corresponding QED theory is due to nuclear effects, which are primarily
the result of the finite nuclear charge and magnetization distributions. This
difference varies from tens to hundreds of ppm. We have calculated the dominant
nuclear component of the 1s hyperfine interval for deuterium, tritium and
singly ionized helium, using a unified approach with modern second-generation
potentials. The calculated nuclear corrections are within 3% of the
experimental values for deuterium and tritium, but are roughly 20% discrepant
for helium. The nuclear corrections for the trinucleon systems can be
qualitatively understood by invoking SU(4) symmetry.
