The primary function of the dog’s hair is as a protective and insulating coating.
But if the eyes are the gateway or the window to the soul, the skin and hair are both gateway and window to the embodiment of an animals inner health and well-being. Shiny hair, that is, a hair coat that exudes a healthy and lustrous sheen, is an indicator of overall health of the animal. In contrast, a dull coat that lacks luster is an indicator that the animal isn’t as healthy as she could be.
Changes in the texture or appearance of a dog’s coat are an indicator of something may be happening inside, but haircoat changes are not specific for any one disease or condition. Dull or brittle hair can be caused by a dietary imbalance, or it may be due to diseases of digestive, hepatic, renal, thyroidal, immune, or parasitic origin. Alterations of hair typically appear rather late in the course of the disease, as hair growth is rather slow; it usually takes at least four weeks of disease progress before changes are noticeable in the hair. And it typically takes four or more weeks before a dietary supplement will have any positive effects on the quality of the hair.
But body changes that accompany hair thinning can be a sign of diseases like Cushings, and many others.