What makes you think it may be a thyroid problem?
We had a boy quite a few years back, bought hopefully to use on my girls, but would he mate them or any other bitch? Not a chance, we took him to a well known breeder/judge who had had many years experience with stud dogs, even she couldn't get him to mate, checked over by our vet, complete clean bill of health, so he was either gay or just not 'bovvered'
we had him neutered and he went to live with my mother-in-law by the seaside.