There is some controvery re: what certain hormone levels should be, particularly T3 levels. If you are gaining weight without sufficient reason and have other symptoms of low thyroid levels, you might want to read this:
http://www.stopthethyroidmadness.com/

If you have several symptoms, read books by Mary Shoman and consider getting more than one opinion before making your final decision on medication, etc. It seems many doctors are not dxing this condition (esp. in women) and even a slight problem, can cause weight gain.