I have a little bit of insight here, have a handful of close friends and family members who have had hypothyroidism. In all the cases I've personally known, they were all normal weight people with active lifestyles who started gaining weight for no real reason, no change in their diet/exercise habits, and being extra diligent in their eating or going to the gym more often wasn't doing anything to help and that's how they knew something was wrong. The weight gain was a symptom. So it could be that the people you know who developed it after they got fat was because it was diagnosed after they got fat