Hemochromatosis is the most common single gene disease in the United States, more common than cystic fibrosis, Huntington's disease, and muscular dystrophy combined.

As many as one in two hundred Americans have the genes for it, yet it is strikingly unknown and frequently undiagnosed. It's a disease of iron overload, in which the body cannot shut off its storage of iron. Healthy people store two to four grams of iron, mostly in their bone marrow, but hemochromatosis leads to the storage of twenty or more grams.

The body squirrels the extra iron away in its vital organs, starting with the liver. Once the liver is full, the iron is packed away in the pancreas, heart, and pituitary gland. Excess stored iron is a toxin that leads to a number of miserable consequences. In the liver, the excess iron causes cirrhosis and liver cancer.

Iron-caused damage to the pancreas leads to diabetes, called "bronze diabetes" because the skin is tinted by the excess iron and has a bronze or gray tone. In a 1999 study, diabetes was found in forty percent of people at the time their hemochromatosis was diagnosed. And some sources recommend that everyone with diabetes be tested for hemochromatosis.

Hemochromatosis is an inherited disease that occurs in people who have two copies of the mutated gene that causes it, one from each parent. Among Irish-Americans and African-Americans, one in every hundred people has two copies of the gene.

If you've inherited only one copy of the mutated gene, you are called a "carrier" because you can pass that mutation on to your children. Thirteen percent of the population are carriers of the mutation. Among Irish-Americans and African-Americans, a full twenty percent are carriers.

Because most people with hemochromatosis don't know they have it, it is sometimes called "the silent killer." The symptoms, including deep fatigue, joint pain, hair loss, and abdominal pain, usually don't surface until the middle years of life, when people tend to blame them on the advancing decrepitude of old age.

If not caught early enough, hemochromatosis leads to diabetes, liver disease, erectile dysfunction, liver and pancreas cancer, jaundice, heart disease, and thyroid disease. The key, therefore, is to catch it before such damage is done. And the best way to do that is with a blood test that, because it is not part of the standard test battery, must be specifically requested by your doctor.

Hemochromatosis is diagnosed by blood tests called serum iron and total iron binding capacity (TIBC) tests. The serum iron (iron in the blood) is divided by the TIBC to give a ratio called the transferrin saturation. (Transferrin is a protein that transfers iron in the blood to the liver, spleen, and bone marrow.)

If the ratio of serum iron to TIBC is over 45 percent, a serum ferritin test is done. (Ferritin is the storage form of iron and directly relates to the amount of iron stored in the body.)

If you have relatives with hemochromatosis, these tests should be part of your yearly health check-up. If caught early enough, the disease can be rendered completely harmless by a program of, oddly enough, old-fashioned blood-letting.

These days it's called phlebotomy instead, but it still consists of draining blood from the body, just like when you donate to a blood bank. Every bag of blood contains between 200 and 250 milligrams of off-loaded iron. Phlebotomies are carried out weekly until the ferritin levels are down where they should be, which could take as long as three years. After that, the process need happen only every few months.