Hyperkeratosis is a product of your body’s defense mechanisms. In this case, the keratin protein in the layer beneath the skin is reacting to irritation of some sort. What happens then is that the keratin will start producing extra layers of skin tissue to deal with the irritation so that the skin thickens to deal with it.
On the feet, you can find corns, calluses and even bunions can be considered the results of hyperkeratosis as the keratin works against the irritation that is occurring. For instance, if your shoes rub your instep, you are likely to start developing bunions as a result. They are just huge knobs of thickened skin that resulted from the shoe rubbing against the area. On a toe, the result is likely to be a corn or callused area. One thing to notice is that the thickened skin is often excess tissue that has built up on good tissue to provide protection.
Bunions, by the way, can occur at any point on the foot where an irritation occurs.
Keeping with the feet, a bit longer, if you walk along a gym shower floor in your bare feet or you use a pool in your bare feet, without shower clogs, then you may find you are the victim of a Human Papillo Virus (HPV) infection, better known as a Plantars Wart. This type of hyperkeratosis is spread by contact and will usually go away after several months.
Another type of hyperkeratosis that may attack the face, arms or legs is often the result of eczema attacks. Also known as dermatitis, unless the itching and skin problems are controlled, the result can also be hyperkeratosis. One of the results of this is skin discoloration.
Oral hyperkeratosis is also known as lichens planus and results from the walls of the mouth feeling constant irritation due to some factor such as a dental device that may be rubbing or one may constantly be biting the interior of the cheeks and a white, lacy pattern develops, as well as thickening. This may require a biopsy to determine the severity of the problem. This is often a problem in chronic users of smokeless tobacco. No one really knows the ultimate cause of this problem, other than the irritation itself but some think that it could also be the result of an abnormal immune response to this issue.