Nancy, I had the same issues, and went through multiple m/c before successfully carrying a pregnancy to term. I saw multiple doctors during my struggle w/ infertility, and many, many doctors do not really understand the impact of thyroid function on fertility and pregnancy. I finally found one who did, thank God. Your TSH should be 0.5 - 2.5. Many labs and doctors will tell you that if your TSH is < 6.5, you're ok. Maybe for staying alive, but not for staying pregnant - at least based on my experience. As soon as you get pregnant, your TSH is going to go up (one time, mine went from maybe 2.2 to over 6 in two weeks, when I confirmed I was pregnant), because your body doesn't make enough, and you have to produce for your baby too. So, you will need to increase your medication dosage as soon as you get a positive pregnancy test. The usual amount of increase needed is 30 - 50% of the dosage you are currently on, but everyone is different and you'll need to be checked to see how your body is reacting. You don't want to go overboard and take too much synthroid, that's not good either.
Here are some resources for you:
www.thyroid.org
There are some patient information brochures on this site
http://www.thyroidmanager.org
This is an on-line textbook of thyroid topics, written by the opinion leaders in the field, and frequently updated. This is not in layman's terms, so if you don't have a science background, you may find it tough going. There is a chapter on thyroid function and pregnancy that is worth slogging through even if you only understand 10% of it.
If your doctor isn't familiar w/ the new guidelines, and isn't willing to read up on this, get a new doctor. He/she is doing you a huge disservice, and you need a doctor who is up on the latest research as there has been a huge amount of research in the last 15 years or so that seems to have changed a lot of long-standing ideas.
I also like http://www.thyroid.about.com/, which is intended for the layperson and has a very good editor (Mary Shoman). I think it was there that I read about taking Synthroid on an empty stomach, and not mixing calcium or iron or other supplements with it. The key is to be consistent, so your body is always seeing a constant dose.
A friend of mine who works for a major pharmaceutical company put me in touch w/ a colleague who works in this area, and he told me "Women who have thyroid dysfunction already should make sure their TSH levels are normalized before becoming pregnant. However, even the concept of "normal" is under review by the scientific community. Nowadays, a TSH of < about 2.5, but > 0.5 is considered "really" normal. People with TSH of 2 - 4 are more likely to have early stages of thyroid failure."
Good luck, sorry this is so long but I had so much info I wanted to pass along to you! You need to be your own advocate. You need to become informed so you can ask intelligent questions. Don't count on your doctor to have a magic wand and make everything right. Medicine is not an exact science, and different doctors know different things.