My dog has made 6 trips from CA to to Mt. Hood, OR which included going through the central valley each time. The central valley is quite hot during the summer. The only time my dog made that trip on Soloxine he had a very bad heat reaction, almost like a heatstroke. That was the 5th trip. I immediately took him off Soloxine and for the 6th trip home he was fine. I really think that although he was in an air-suffered from the side effect of Soloxine which is an intolerance to heat. Usually that side effect isn't mentioned but I did see it mentioned a couple of places.
I took him off Soloxine for the summer and have put him back on as the weather has cooled for the fall.