Slow weight loss on Phentermine

I am 2 weeks in and only have lost 3 lbs despite working out 5 days a week and reducing food intake and serving size. I was prescribed 18.75mg for the first month, I was hoping to be further along by now. I am trying to lose 20lbs. I have a thyroid issue so being on it has definitely given me the energy that I desperately needed. I am just wondering if I will see more results once I increase dosage?!??