Yes, your TSH level is inversely related to thyroid function--i.e., low TSH means hyPERthyroid (overactive), while high TSH means hyPOthyroid (underactive).
But there is SO MUCH MORE to properly evaluating thyroid function than merely the TSH--even though most conventional doctors AND endocrinologists still rely heavily or entirely on the TSH.
I have Hashimoto's, which means that my thyroid can swing from overactive to underactive periodically, which means I need to stay on top of evaluating it so that I can adjust my meds accordingly.
Like PP mentioned, definitely spend some time checking out the http://www.stopthethyroidmadness.com/ If anything, learn about the other lab tests, besides just TSH, that you should INSIST your doctor run, which are far more indicative of actual thyroid function. And learn about natural dessicated thyroid, like Armour and Naturethroid, that treat thyroid dysfunction far better than Synthroid (the synthetic brand that nearly all conventional doctors prescribe).
MOST IMPORTANTLY, find a doctor who will treat you based on SYMPTOMS, not just LAB TESTS. I learned that early on--my first doctor refused to continue treating me after he got my lab tests within "normal" range, even though I had experienced NO improvements in my actual symptoms. That's pathetic, but all too common. There are definitely doctors and naturopaths out there who will treat based on symptoms, and that's what you want.
Like you said, OP, it's true that some women get post partum thyroiditis, which does often resolve itself. But it's also true that if you have any existing thyroid issue, and then you get pregnant, your thyroid levels almost always get even more out of whack within the first several weeks of pregnancy. And you MUST address the thyroid issue asap, because proper thyroid function is imperative for the embryo during the first trimester.
All of my thyroid docs (even the crappy ones) have said that I need to get my thyroid tested within 3 weeks of getting a BFP, even if I had just had my normal thyroid tests done a week before the BFP--because a new pregnancy can shift thyroid function that quickly, and it can be very dangerous to the baby.
Sorry for the novel :) Since my diagnosis two years ago, I made it my full time job to research this stuff......and I'm still faaaarrrrr from an expert.