Hair loss in dogs has many possible causes, including mange. Hair loss combined with darkening of the skin can be caused by Cushing’s disease, hypothyroidism and some chronic skin infections. Cushing’s disease would cause additional symptoms such as increased drinking and urination, panting and high blood pressure. You should take your dog for a thorough examination by your vet, who will be able to diagnose Cushing’s disease, hypothyroidism, or a chronic skin infection.

This combination of hair loss and skin darkening can also be indicative of an uncommon and not-well-understood condition called "Black Skin Disease". Other names for this condition include "Wooly Coat" and "Coat Funk". Breeds most commonly associated with this condition are the Pomeranian, Alaskan Malamute, Poodle and American Water Spaniel. I do not have any information at hand about the occurrence of this condition in Golden Retrievers. In Pomeranians the condition is suspected to be an inherited disorder on the X-chromosome because it is normally found only in females, but little is known of its causes in other breeds. It is often assumed that affected dogs have growth hormone responsive alopecia (hair loss), but many affected dogs do not suffer from this. Because of this, the condition is not associated with a specific set of hormone changes, however the condition often clears up after neutering.