I've been taking iodine but I haven't had any reactions like you said people do. Is that a bad sign? Does that mean I'm not deficient?

No it doesn't mean you’re not deficient. You could still be deficient. 

I see plenty of people who are deficient but then they don't have any reactions. It comes down to genetics. Some people have reactions, while others do not. But all of them can still be deficient.

So if you do have a reaction, it means you are definitely deficient. 

But if you don't have a reaction, you can still be deficient.

Not having a reaction is not a bad sign. Consider yourself lucky.