[observer]

I have a 12 year old right now. She has been on thyroid meds since before she was 9. My previous Rottie was on the same meds for about 3 years, died of cancer at 11.

What you see as him being slow and tired could be one sign of thyroid, but it's just one. Has he gained weight? How is his coat? Do his feet seem cool/cold to the touch? Any of those things can be present in a hypothyroid dog. They don't all have to be. He may just be slowing down, without any real pathology to explain it, but if you are really concerned, I would have him checked with a full thyroid panel.