Dark Spots

Question
My 10 year old Chihuahua's skin has started to develop some dark patches. It first began on her sides but has traveled to her abdomen only recently. This has been going on for about a month or so I'd say. The spots don't seem to be tender when I touch them and she doesn't seem bothered by them. They aren't raised either. She's tricolored but her skin is usually pink. She is still growing hair back on her belly because she had surgery a few months ago and recovered just fine from it. She isn't lethargic, continues to be energetic and want to play. She still enjoys going on walks as well even though she's getting up there in age. I haven't really seen any other changes with her so these spots have me a bit concerned. Some have told me that it could possibly be because of old age. I guess I would just like some peace of mind!

Answer
Many older dogs will develop pigmented areas on their skin with age.  If there was an insult to the skin (like surgery) then the skin can also pigment.  Some dogs with allergies will also develop dark skin on the belly. Dogs with low thyroid hormones will develop thickened skin often darker than before.