Hypothyroidism and Low t4 during pregnancy?

I am 28 weeks pregnant and have been experiencing rapid heart rate and shortness of breath. My OB tested my thyroid and my T4 levels came back low. He has referred me to an endocrinologist but my appointment isn't for a week. They said this def. could be causing my symptoms (my pulse jumps to 130-150 for no apparent reason). Earlier in my pregnancy my hair would fall out. They told me it was just the hormones. I became frustrated with my OB because she would tell me everything was fine and I knew there was something wrong. I switched to my Doctor and in about 2 weeks he had the problem figured out. My concern is that I am already 7 months pregnant and I know that this condition can have some effect on the baby. Could this have been going on the entire pregnancy and the one before that? I did have a hard time with my weight every since I hit puberty. I joined the swim team in high school thinking it would help me. We were swimming 10000 yards a day and I lost no weight at all. Could this have harmed my baby? Anyone have any experience with this?