Hyperkeratosis is a thickening of the outer layer of the skin, which contains a tough, protective protein called keratin. This thickening is often part of the skin's normal protection against rubbing, pressure and other forms of local irritation, and causes calluses and corns on hands and feet or whitish areas inside the mouth. Other forms of hyperkeratosis can occur as part of the skin's defense against chronic (long-lasting) inflammation, infection, the radiation of sunlight or irritating chemicals.