first thing...don't panic. It could be, as alaysha said, a number of things. brown blood is definitely better than red. having said that, i've bled red and baby has been fine. And remember, it's only spotting, not lots of blood. the fact you have no pain is a good sign. you do need to see your dr though to check it out.
I also forgot to mention that my sister has hyperthyroidism and has never had any bleeding or spotting during any of her pregnancies so I doubt that is the reason.
I agree with alaysha. If you have to see bleeding, brown is definitely better then any kind of red or pink etc. like she said it's old blood. I don't know what if anything hypothyroid would mean for a pregnancy so yeah I agree with alaysha Call in to your doctors office if for no other reason then getting a little reassurance. good luck!
Lots of things can cause bleeding or spotting during early pregnancy. The best thing to do would be to contact your doctor and get checked out to make sure everything is ok. Since it is brown blood (old blood) I wouldn't think it would be anything too serious. Maybe your body shedding the extra lining in your uterus.