Halo is 14 now (lab X) and she pants constantly. She has been diagnosed (as it were) with atypical Cushings disease. Apparently panting and excessive drinking are common symptoms of cushings. Not trying to scare you, just saying. We have heavy panting all the time. I spent a medium sized fortune trying to get a diagnosis of cushings (ultra sound, blood work) and in the end the adrenal glands were enlarged - as shown in the ultra sound - but the cushings blood test came back normal, hence the diagnosis of atypical cushings. For a further $600 dollars we could have sent more blood to the US, but enough is enough.
So just throwing that out there as it may explain some of the panting, cushings of course is common in older dogs.
Our Malamutes are panting heavily also with the heat - when they're not just passed out!
I hope Timber is doing better now.