There is a disease called "Black Skin Disease". Symptoms of dog black skin disease include severe hair loss and darkening of the skin (hyperpigmentation). Dog black skin disease often begins with loss of hair.
Some breeds are more susceptible to the condition than others, including the Alaskan malamute, poodle, chow chow, and Pomeranian. Male dogs are more susceptible than females. Which your GS doesn't fall into this, however, it doesn't rule it out.
This disease can be caused from low levels of growth hormones. Your vet can determine this through a blood test. This can also be related to Thyroid or Cushing disease. Your vet can help you determine what is going on and start treatment.
Dog's that are neutered or spayed (although you didn't mention if yours is or not) will benefit if they have this disease.
I hope this helps and your German Shepherd is feeling better very soon. Please let us know if you have any other questions or concerns. ~Victoria~