Alopecia is partial or complete lack of hair in areas where it is normally present. It is basically classified into congenital and acquired alopecia, and etiologically divided into functional, structural and traumatic events.

Congenital alopecia commonly encountered in dogs is color related alopecia such as color dilution alopecia (CDA) and Black hair follicular dysplasia (BHFD) as well as pattern alopecia. Hair loss appears at the diluted color hair coat (i.e., blue) in CDA and the black hair coat in BHFD. The hairs are normal at birth but gradually begin to lose. Microscopic and histopathologic examinations demonstrate crude clumped pigments within the affected anagen hairs, which could break resulting in alopecia. Pattern alopecia (Pattern baldness) is idiopathic that often begins during late puberty or early adulthood. It is most common in short-coated breeds, and symmetric hair loss locates at the pinnae, ventral neck and trunk, and/or posterior thigh. Histopathologic findings are characterized by miniaturized hair follicles.

Acquired alopecia is apparently higher incidence compared to congenital alopecia. Among those, functional alopecia, which is caused by the alteration in hair cycling, leads symmetrically generalized or regulated alopecia without inflammation. The alopecia with systemic symptoms suggests endocrine disorders, as well as anagen effluvium and telogen effluvium in some. While the alopecia without systemic symptoms suggests age-related and breed-related phenomena, in particular "Alopecia X", seasonal flank alopecia and so on. Structural alopecia is loss of the normal follicular structure, which generally shows non-symmetrically localized or nonregulated alopecia with inflammation. The most common cause is infectious folliculitis including Staphylococcus folliculitis, demodicosis, and dermatophytosis. Traumatic alopecia is fracture of the hair shaft due to mechanical stress, as well as some infection to the hair shaft such as Microsporum canis. Broken hair shafts are usually visible as the short stubble of hair on the surface on the skin. Microscopic examination can easily demonstrate broken hairs at affected areas.

Major endocrine disorders featuring hair cycle related alopecia are Cushing's syndrome (hyperadrenocorticism), hypothyroidism, and sex hormone imbalance. Alopecia due to endocrine disorders is commonly seen in middle-aged to senile dogs, and each disease may show some specific findings. It could be clinically suspected, but definitive diagnosis always requires blood tests, hormonal assay, urine test, image examination and/or therapeutic evaluation because hormones interact among their network. Anagen effluvium is loss of anagen hairs due to impairment of mitotic activity. The classic cause is systemic chemotherapy, especially alkylating agents (e.g., cyclophosphamide), which leads dramatic hair loss 1 to 2 weeks after the exposure of the agent. Telogen effluvium is a reaction pattern to a variety of physical, physiological, or mental stressors as well as impaired nutrition. It leads to generalized hair shedding and a diffuse reduction in hair volume, but the shedding is never so severe as to render bald. It appears a few months after the inciting event. In general, the shedding of both anagen and telogen effluvium resolves spontaneously if the cause is removed.

"Alopecia X" is a progressive hair cycle related alopecia without any systemic signs and inciting events. The highest incidence is in intact male, young adulthood, especially Pomeranians, Siberian huskies, Malamutes, Samoyeds, Keeshonds, and miniature and toy Poodles. Loss of primary hairs progresses to complete alopecia of the neck, and caudal thighs. Alopecia eventually becomes generalized on the trunk, but the hairs of the head and extremities remain. The etiology is uncertain, but several theories have been proposed; hyposomatotropism in the mature dog, gonadal sex hormone imbalance in intact male dog, and adrenal sex hormone disorders. Interestingly, hair regrowth can be observed following trauma, mechanical stimulation such as skin biopsy. It is suggested that some key player for hair cycle presents in the skin in association with systemic endocrine system.

In human, there is a male predominate and non life-threatening, progressive pattern hair loss from the scalp, which is described as androgenic alopecia (AGA). Hair loss is characterized by a progressive decline in the duration of anagen, and it is believed that AGA is a process dependent on dihydrotestosterone (DHT). Recently, oral finasteride is used for the treatment of the alopecia. It is an inhibitor of type 2 5αreductase, which blocks the conversion of testosterone to DHT. It can produce visible hair growth in up to 66% of men with mild to moderate alopecia, and it can stop hair loss in 91% of patients. From similar clinical point of view between AGA and Alopecia X, Osaterone acetate, oral active steroidal antiandrogen for canine prostate hyperplasia, was evaluated in dogs with the alopecia. Of 13 dogs, 7 dogs (53.8 %) showed clinical improvement. Overall, it slowed the regression of hair loss and increased hair growth in 76.9 %. No side effects were reported. It is considered that this present alopecia corresponds to AGA in humans, and further investigation will be warranted.