The new guidelines can be found at http://www.aace.com/clin/guidelines/hypo_hyper.pdf. The numerics are on page 462 of the journal article, page 6 of the PDF. The new levels for TSH are 0.3 - 3.0. TSH works inversely to your thyroid levels - the higher your TSH is, the more hypothyroid you are. My levels were testing right around 5.5 as early as 6 or 7 years ago, but I wasn't diagnosed until late last spring. (I hadn't mentioned it to my current doctor, since I had had so many doctors tell me that, yes, my thyroid was normal, all my symptoms must be due to something else, so I had gotten used to just ignoring the symptoms.)
It can take a while to get your thyroid medication levels ironed out, so I would go ahead and have them checked to be safe if you are concerned at all that you have hypothyroidism. They can't even do your first set of levels after you are on your medication until about six weeks after the test. Make sure your doctor is aware of the new guidelines, too - my doctor's nurse was not aware of them, and when she called me with my first set of results she thought they were within range when they were still a little bit out. (My doctor's nurse handles stuff if I call before my doc has a chance to review my labs - my doctor had already told me about the change in the accepted levels when I had the test done.) We've been bumping my levels a notch every couple of months for the past year, and they are finally staying at the lower end of the normal TSH range.
If you are diagnosed, it will be very important to stay on top of your levels - have them checked at least once every trimester. Both hypothyroidism and hyperthyroidism can cause preeclampsia. My doctor told me how often she was planning on checking mine during my pregnancy, and it sounded like it would average out to about every 6-8 weeks, more often if I am out of range.
I around the age of 2 is a good time to TTC - we were going to start trying when Rhys was a little over 2, but we wanted to get my thyroid straightened out before we started trying. Rhys is getting the hang of potty training, so a bonus to waiting is that we will have a few months with no diapers, but I would have liked to have them a little closer together in age.
Whew! Can I ramble or what?!