For reasons as yet unknown, not all individuals who are homozygous for the C282Y mutation display phenotypic features of HH, and persons with H63D polymorphisms rarely develop iron overload. The penetrance of HFE mutations is generally considered to be low.
Results of a recent meta analysis by the US Preventive Services Task Force conclude that 38% to 50% of all C282Y homozygotes develop some evidence of iron overload, but that only 10% to 33% develop clinical disease due to HH. (11) In other words, some individuals may have elevated iron test results such as transferrin saturation, but do not demonstrate significant organ damage.
Estimates of penetrance in some studies are even lower. Penetrance of HFE mutations is currently a controversial subject among experts, and the significance of finding HFE mutations in a given individual is often unclear. The probability that a given individual with HFE mutations will develop clinical disease from iron overload cannot be determined at this time.