Will I continue to lose weight with my thyroid condition?

About a year ago I started gaining weight rapidly despite a healthy diet and exercise. I also had extreme fatigue, constipation, stiff joints, muscle weakness, brittle fingernails and hair. A few months ago I was diagnosed with hypothyroidism. My doctor prescribed Synthroid and within 6 weeks, I’d dropped 15 lbs.

Does this mean I will continue to lose weight? He says my thyroid levels are normal, does that mean my metabolism is working correctly? I’ve tried to find information on this, but I just can’t find exactly what I’m looking for. I still need to lose about 50 lbs, but I’m still having issues with fatigue and it makes it extremely difficult to exercise.