In this condition an abnormality of the pituitary gland leads to a lack of growth hormone (and possibly other hormones such as thyroid hormone and adrenal gland hormones). This causes a failure to grow, and retention of the puppy coat. 
In German Shepherds this is an uncommon condition that is inherited as an autosomal recessive trait.  In Australia it has been reported that up to 20% of the population may be carriers.  A DNA test is available for this condition, and affected puppies are easily recognised, and generally euthanised.  Treatment of some dogs has been attempted with symptomatic therapy and depo-progesterone.  In general growth hormone is not commercially available, and would only reverse the dwarfism if given very early in the course of the disease. Pituitary dwarfism is easily distinguished from osteochondrodysplasia, sometimes called chondrodysplastic dwarfism, which is a congenital condition affecting the long bones of the legs, resulting in very short legs.  The rest of the dog is relatively normal in appearance with chondrodysplastic dwarfism.