Hair loss may occur through several mechanisms, however the only form related to testosterone levels is known as androgenetic alopecia. A hereditary disorder characterized by thinning of scalp hair, androgenetic alopecia occurs in both men and women through an androgen mediated mechanism. While this syndrome is technically a low testosterone state, in fact the body is experiencing high androgen levels due to the mechanism of dihydrotestosterone (DHT), a metabolite of testosterone with potent androgenic effects.
In a healthy hair follicle, circulating DHT binds androgen receptors located on hair follicles. DHT has a potent effect on these receptors, directly inhibiting the growth of the hair follicle. This occurs in normal individuals, allowing accurate control of the hair growth cycle.
Hair Loss Pathogenesis
Circulating testosterone is converted peripherally to DHT by the enzyme 5-alpha reductase. Patients with elevated 5-alpha reductase levels may develop a high DHT/low testosterone state. Pathologic hair loss occurs when the individual's DHT reaches levels at which the inhibitory effects outweigh the stimulatory forces on the hair follicle. Therefore, while low testosterone is associated with hair loss, the pathologic mechanism occurs via the elevated DHT levels. This results in progressively shorter, less pigmented hair production, which ultimately presents as hair thinning and loss.
Presentation and Progression
According to an article from "CURRENT Diagnosis and Treatment in Otolaryngology," androgenic alopecia first presents in men with "bitemporal hairline recession followed by thinning of the vertex." The progression involves further thinning of the vertex and the formation of a bald patch, which may enlarge and ultimately combine with the frontal receding hairline. Female hair loss typically presents with diffuse thinning of the frontal or parietal scalp, though a rim of hair along the frontal hairline is often maintained.
Researchers at St. Vincent's Hospital Melbourne Department of Dermatology reviewed the advancements in treatment of androgenic alopecia. The therapy has approached the treatment of this disease based on our knowledge of its pathogenesis. In the article, Dr. Rathnayake reports on inhibitors which target 5-alpha reductase, thus reducing the overproduction of DHT in patients. As of November 2010, these drugs are in phase III trials.
Prevention and Treatment
State-of-the-art treatments for hair loss related to decreased testosterone include topical minoxidil and oral finasteride. These treatments are FDA approved for the treatment of androgenic alopecia. While rather effective in reducing the progression of the disease, these treatments allow only partial re-growth of hair that has already been lost.