Hirsutism is a term which refers to excessive or abnormal hair growth.  Just as baldness is a blanket term which may include pattern baldness, alopecia, or trichotillomania, "hirsutism" is a blanket term which applies to various conditions resulting in excessive hair growth.  Moderate and severe hirsutism is referred to as hypertrichosis, and may include localized hypertrichosis (abnormal hair growth on one specific part of the body), or generalized hypertrichosis (abnormal hair growth on the entire body).  Hypertrichosis can also be congenital (present from birth), or acquired (developing over time).

Since there are many different types of hirsutism, there are also many different causes of hirsutism.   Here are some of the most common causes of hirsutism or hypertrichosis:

Hormones:  Testosterone and estrogen are the two hormones which most directly impact hair growth.  These hormones are present in both males and females to varying degrees.  Mild hirsutism can often occur in older women, since females produce higher levels of testosterone as they age.  Hormone replacement therapy may be a viable option for those who suffer from mild hirsutism caused by hormonal imbalance.  Mild hirsutism can also be treated by shaving, waxing, or electrolysis.

Genetics:  Although genetics account for a relatively small percentage of those who suffer from hirsutism, it is one possible cause of hirsutism or hypertrichosis.  Some forms of generalized congenital hypertrichosis, for example, are often linked to genetics.  Since there is no "cure" for genetic hirsutism, treatments often include shaving, waxing, or electrolysis.

Medications:  There are many drugs and medications which result in hypertrichosis.  Some drugs, like minoxidil, are specifically used to grow hair.  In other drugs, hirsutism is an unwanted side effect.  Drug-induced hirsutism has been linked to the following drugs and medications: acetazolamide, cyclosporine, diazoxide, latanoprost, psoralen, and streptomycin.

Porphyria:  Porphyria is a systemic disease which often occurs with exposure to certain chemicals.  Hexachlorobenzine, which is a chemical fungicide sometimes used in agriculture, is one such chemical which may lead to hepatic porphyria.  Hirsutism is a common symptom in some forms of porphyria, including hepatic porphyria.

Injury:

Some experts believe there is a link between cerebral injuries and some generalized forms of hirsutism.  Until recently, it was believed that phenytoin, a drug used to treat brain injuries, was responsible for this type of hirsutism.  However, recent studies have found that generalized hirsutism is common in patients suffering from brain injuries who have never taken phenytoin.


Malnutrition:

 Low-carbohydrate diets have been linked to vellus hypertrichosis, which affects the growth of hair on the arms and legs.  This form of hirsutism has also been linked to anorexia, bulemia, and other eating disorders.
Cancer:  Specific types of hirsutism, such as localized acquired hypertrichosis, can be a symptom of some forms of cancer.

With so many various causes of hirsutism and hypertrichosis, it is difficult to find a "cure" to this condition.  In some cases, such as acquired hypertrichosis, hirsutism may even indicate a more serious health condition such as cancer or porphyria.  Therefore, before beginning any treatment for hirsutism, it is a good idea to seek professional medical advice.

Author's Note:  A very extensive and informative guide on the various forms of hypertrichosis can be found online at: http://www.hypertrichosis.com
Sources:  Hordinsky, Maria K.; Amy J. McMichael (2008). Hair and Scalp Diseases: Medical, Surgical, and Cosmetic Treatments (Basic and Clinical Dermatology).
Clark, Jennifer (October). http://dermatologytimes.modernmedicine.com/dermatolo gytimes/issue/issueDetail.jsp?id=10462
Milady's Standard Textbook of Cosmetology (Milady Publishing, 2000).