Apparently he had been suffering from Cushings Disease.
There is a link to a detailed description of the condition in the article.
The officer said that this dog was his best friend, which I know all to well how hard it is to put your best friend down, so I trust that this decision was made very carefully and lovingly.
Cushings... The “barn hens” where we board the horse, and the wife works, cackle that phrase every time a horse has a sniffle or slight oddity.