Lethargy, anorexia, panting, hind leg weakness, gastrointestinal problems, vomiting
, and hair thinning, were the symptoms that forced you to take your <span style = 'background-color: #dae8f4'>dog</span>
to the vet initially. These symptoms can be signs of endocrine disorders, specifically Hypoadrenocorticism (Addison's Disease), Hyperadrenocorticism (Cushing's Syndrome), and hypothyroid.
I think that your dog
may have had undiagnosed Addison’s disease (Hypoadrenocorticism).