So what IS the normal thyroid medicine dosage???

Ok, so a few months ago, I went to the gyno because my husband and I want to have children. But here's the thing. I don't have a period unless I'm on Birth Control (which kinda defeats the purpose when I'm wanting kids). The doctor took some blood so she could run a lab on me. Turns out that my thyroid level was low, and I have PCOS (PolyCystic Ovarian Syndrome), hence, no period, no ovulation, no kids. Anyways she sent me to an endocrinologist, and he put me on Levoxyl 125 mcg. My grandmother (who also has hypothyroidism) said she's only on 50 mcg. Now my Endo wants me to go up to 150 mcg, because after 2 months, my thyroid is still low, still no period (I know most women would love this, but I haven't had one in about 3 years) and still not ovulating... (grrrr.... ) Anyways, I was just wondering, what's the normal dosage? (Oh, btw, I never even dreamed that I was having thyroid problems. I didn't feel unusually fatigued, no unusual weight gain, no decreased sex drive, no nothing... Although, I guess that I could just be used to all of those things, as I've always been able to sleep at the drop of a hat, I've always been big, and I've always battled with depression. I thought that was normal...)