It is a good idea to get this checked out by a vet as there are a number of things that can cause hair loss as you describe, some potentially more serious than others and each of them may require a different form of treatment.

Hairs grow in cycles, and each cycle consist of a growing period and a resting period. The hair production cycle is highly variable in dogs and to a large extent breed specific. However most breeds fall into a six-month cycle with two hair sheddings per year, in early spring and autumn. Dogs that are housed indoors can be exposed to several hours of artificial light, which means they may shed their hairs throughout the year, since their cycle is regulated by sunlight.

Reasons for incessant molting can include congenital Follicular dystrophy with which there is abnormal development of the coat; Hair cycle arrest which can be associated with many diseases associated with adrenal glands and other regulatory systems; and can also be caused by hormonal changes due to stress or anxiety. Aside from medical reasons, hair loss can also be due to behavioral problems such as constant scratching.