In all but the most severe cases it accounts for a gain of about 10% of your bodyweight (it's zero mystery if you have the really bad kind that can cause more, because there's a slew of other really obvious symptoms that require serious medical intervention). A normal person's reaction to learning of having hypothyroidism would be, "Oh man, you remember when I seemingly inexplicably went up a couple pants sizes? Well terrible new..." A fats reaction? "GREAT NEWS! It's not my fault I weigh 400lbs because 15 of them can be explained by a medical condition!"