A  Hair growth occurs in cycles and is influenced by photoperiod and, to a lesser extent, ambient temperature. Hair growth is regulated by a complex interaction of the hypothalamus and hypophysis of the pituitary gland (in the brain) to regulate multiple hormones, including melatonin, prolactin, thyroid hormone, and cortisol. The exact signals to the equine hair follicles that induce active hair growth (anagen), the transitional stage (catagen), and the resting stage (telogen) are not known. The length of time the hair spends in each of these stages is dependent on the region of the body, age, breed, and sex. The growth of hair can be modified by a variety of physiologic or pathologic factors.

Hair cycle stages and, thus, hair growth and loss, occur in a mosaic pattern in horses, meaning adjacent hair follicles are in different stages of growth at the same time. Horses in temperate climates generally shed the long coat of winter over several weeks to the shorter haircoat of summer. But occasionally horses will have a less-coordinated transition from a seasonal coat (winter to spring, fall to winter), during which more hairs are shed in a period of time than new hair growth occurs, leading to a very thin coat or, very rarely, to areas of alopecia (baldness). In this condition the skin is normal and a skin biopsy reveals normal hair follicles. Over a period of several weeks the new hair growth "catches up" with the hair loss, and the coat is normal.

Hair loss, either patchy or total hair coat, can also occur following periods of stress (such as pregnancy) or generalized illness in which the growth period of the hair is considerably shortened  and many hair follicles enter the resting stage at one time. Patchy or generalized hair thinning or alopecia can occur four to 12 weeks following the stress