You don't quite seem to understand how physics work. Genes do not create lard. Disease does not create lard. Fat does not spontaneously pop into existence.

Do you actually understand what metabolism is? It's the rate at which your body consumes energy. A more efficient body stores it better and spends it slower, meaning it takes less calories to gain weight. Your genes are not capable of injecting lard into your ass. You have to eat. If you have a slow metabolism you do not require as much food energy as other people. If your energy consumption decreases accordingly, you still won't get fat.

Hyperthyroidism release too much of a hormone that controls your metabolism you know, that thing I just explained. It produces an excess of a hormone, it does not produce fat. You still have to be eating more energy than your body spends to store fat.

Medicines can have an effect on metabolism. They can not materialize fat from nothing.

Steroids are the one thing you've brought up that might actually be a relevant criticism, as prednisone in particular can greatly increase liquid retention, increasing your size and weight without actually adding any fat. Of course prednisone is also known to increase appetite, and most people taking it have trouble with physical activity, so you've got a two pronged problem there.

Ammendment: If you don't have energy to store, you don't gain fat.

I won't indulge in the insult game, primarily because with a troll as gifted as yourself, I honestly can't tell when you're serious. I'm just going to assume you're not.