My Oscar eats everything from dog food to dumplings to ice cream to chocolate to water melon to crossant, and he looks just like any other dogs strictly on dog food diet.
The majority of his food intake is still dog chow. Junk food only consists of a tiny part of his diet. According to the vet, it's highly unlikely that he's not getting enough nutrients from today's brank name dog chows.
Hypothyroidism is largely genetic, some breeds like Golden Retrievers or Doberman Pinschers are more prone to this disease.