In young people, hair gets its characteristic blond, brown or black color from varying amounts of a pigment called melanin, the same pigment that colors skin. But with normal aging, hair follicles produce less and less melanin, allowing the underlying white color of hair to become apparent. A head of hair looks gray when some hair has turned white and the rest is still pigmented.

Some auto-immune problems, in which antibodies mistakenly attack parts of the body, including melanin-producing cells in hair follicles, can also lead to gray hair. If a person has Hashimoto’s disease, for instance, in which antibodies attack the thyroid gland, he or she may also have other antibodies that attack melanin-producing cells in follicles, said Dr. Adrian Dobs, an endocrinologist at Johns Hopkins University.

Another auto-immune problem called alopecia areata, in which hair falls out, can also appear to make a person go gray rapidly, said Dr.Howard Baden [cq], a dermatologist at Massachusetts General Hospital. “In this disease, colored hairs fall out while white hairs may not be affected. If that happens, you would turn gray rapidly, not because of a change in pigment but because the darker hairs fell out,” he said. When hair grows back, it comes in white.

Although age is the greatest predictor of gray hair, some people do turn gray “prematurely,” and sometimes this tendency runs in families, said Dobs.

There are only two ways to deal with gray hair. Love it or dye it.