There are a number of causes of hair loss in ferrets.
Seasonal molting
Photoperiod, the number of hours of light per 24-hour period, affects ferrets dramatically. The most obvious effect is molting. Every spring, the ferret will shed out his fluffy winter coat, and grow a sleeker new one. In the fall, the summer coat is changed for a winter one. The ferret will lose weight in the spring, and gain it in the fall, preparing for winter. This response to photoperiod is mediated by melatonin, a hormone produced by the pineal gland in the brain, which affects the hormones released by the pituitary gland. Pituitary hormones have broad effects which include control of estrus cycles in females and testicular development in males. Photoperiod may have an influence on the development of adrenal gland tumors.
Hair loss due to molting is usually gradual, but in the spring, some ferrets shed their whole coat overnight, leaving them with almost no hair for several days. Sometimes the guard hairs (the longer surface hair with the distinctive color) are shed first, leaving only the woollier, pale yellow undercoat. The hair may come out in patches, giving the ferret a moth-eaten appearance. This is normal, and within days, shiny new guard hair can be seen coming up through the undercoat.
Rat tailed ferrets (tail alopecia)
Some ferrets lose most or all of the hair on their tail every summer. This phenomenon, called 'tail alopecia,' is most common in males. The tail begins to look like a rat's tail, with scaly skin, sparse, bristly hair, and blackheads. This is a very unattractive but harmless condition with no known cause. Many nutritional, medical, and dermatological remedies have been tried, and sometimes the hair grows back, with or without treatment. Usually when the ferret changes his coat in the fall, the tail hair regrows, but he is likely to lose it again the next spring.
Hormonal imbalances (endocrine alopecia)
Hair loss caused by hormonal imbalance has a distinct pattern. The hair thins at the base of the tail and inside the legs first, then gradually is lost over most of the body, often sparing the tip of the tail and the head. The common causes are adrenal gland tumors or prolonged heat periods. Jills in heat will grow the hair back when they go out of heat or are spayed. Ferrets with adrenal tumors may develop other more serious problems. If your male or spayed female ferret begins to show hair loss in the described pattern, take your ferret to a veterinarian.
Regrowth of clipped hair
Because the ferret's coat is so sensitive to photoperiod, it may not grow in for a long time after being clipped for surgery or other medical treatment. If the ferret is in a rapid hair growth phase, as in early winter or late spring, the hair will regrow completely in only a few days. If the skin is shaved at other times, such as mid-summer, the area may remain hairless for weeks or months. Just before the hair of dark-colored ferrets begins to come in, the skin will turn black or dark blue, alarming owners who have not seen this happen before. It is just a sign that the hair follicles are making pigment, and in only a few days, the new coat will begin to grow.