This has not been proven, to the best of my knowledge, but these breeds have high tendencies to have antibodies against thyroid hormones and also have higher than average incidence of hypothyroidism. (vetinfo.com)
(18) Many breeds are also reported to have a higher than average incidence of affected individuals, which may also point to a genetic component. (italiangreyhound.org)
Breeds that have an above average incidence of problems include Shelties and Retrievers (Golden, Labrador). (dog-health-handbook.com)