This woman has androgenetic alopecia, which can affect both men and women. Hair loss (alopecia) is generally divided into two categories: scarring and nonscarring. Distinguishing between the two types is important because hair loss that occurs with scarring alopecia is permanent, whereas the hair loss that occurs with nonscarring alopecia is usually reversible, although the hair does not always completely regrow.

Androgenetic alopecia is categorized as a nonscarring alopecia. The prevalence of androgenetic alopecia in white men is 50% at age 50 years and over 70% at age 70 years. The prevalence is lower for women. In men, the temples and vertex are often affected. In contrast, in women the top of the head is affected, and balding is not complete. Androgenetic alopecia results in hair follicles that are thinner in caliber or “miniaturized” and, ultimately, the loss of hair follicles; however, androgenetic alopecia causes a decreased density of hair but not areas of complete loss of hair as seen in alopecia areata. A classic examination finding is “widening” of central part compared with the occipital part as a result of the decreased density of hairs. On examination, there is generalized rather than localized hair loss and decreased density of hair rather than complete smooth patch of hair.

Alopecia areata is an autoimmune disease characterized by well-demarcated areas of nonscarring total alopecia. The involved areas are often circular or oval but may coalesce to form more extensive areas of involvement. The affected areas are devoid of erythema or scale and are asymptomatic. Tapered “exclamation point” hairs may be seen at the periphery; these hairs have shafts that are thicker at the distal portion and narrower near the scalp. This patient's diffuse hair loss is not consistent with alopecia areata.

Frontal fibrosing alopecia is a scarring alopecia and is considered a variant of lichen planopilaris. The frontal scalp shows a band pattern of hair loss associated with follicular hyperkeratosis and perifollicular erythema. This patient's distribution of hair loss is not consistent with frontal fibrosing alopecia.

Telogen effluvium is a form of diffuse, nonscarring alopecia that is usually triggered by a systemically stressful event such as a serious illness, surgery, or childbirth. It is most commonly seen in women in the postpartum period. It is caused by the realignment of a higher-than-average number of hair follicles into the telogen, or final, phase of hair development. Approximately 3 to 5 months after the inciting event, numerous hairs are shed, leading to noticeably thinned hair. A small hair bulb may be visible at the scalp end of the shed hair. The hair loss tends to be patchy and diffuse; foci of total alopecia are not seen, and inflammation is absent. In most patients, the follicular maturation cycling returns to normal after a few months. This patient's history of gradual hair loss over 2 years is not consistent with telogen effluvium.