Hi
As long as the collar doesn't press against his throat/thyroid, but I wouldn't know how to ensure that. It depends on how it fits around his throat, I think.
Just out of curiosity, is your cat currently being treated for Hyperthyroidism? And if so, what type of treatment is he or she getting? Since our cat was also diagnosed with H-T and has gone through treatment I would suggest contacting your vet and asking them about the pheromone collar. Good luck!