Giving thyroid hormone as a weight loss aid was "popular" back in the 1970's ... but was stopped when they realized that giving it to someone who wasn't actually hypothyroid would cause the thyroid gland to stop functioning PERMANENTLY, thus
causing the very disease it was intended to "help". As a consequence, doctors are very reluctant to prescribe, until blood and lab tests are done to confirm a diagnosis of hypothyroid.
Failure to lose weight when dieting is a concern IF you have followed the diet to the letter, without cheating -- or without undereating and trying to starve yourself, or without bingeing. According to your stats, you've already lost 133 lbs by following Atkins .. that's quite an achievement for 18 yrs old. You should have the diet memorized by now, and be at the point where you're able to help others and offer suggestions so that they can be as successful as you have been.
Perhaps you're closer to your goal weight than you think. You're 5'8" .. 120 lbs may be unrealistically low for you. Are you exercising? This will definitely be a boost to your metabolism, and should help your stall problem as well.
My suggestion is that you get right back to basics, and follow Dr. Atkins Induction AS WRITTEN in the book, for at least two weeks. Follow the sample menus exactly, and eat only the foods that are listed there, nothing else. No cheating! I think you will be very pleased with the results.
Good luck
Doreen