low thyroid during pregnancy

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I am 18 4/7 weeks pregnant. In the past I had a miscarriage at about 8 weeks pregnant. I have 12 year old boy. Last Friday I received a letter from my OB saying my thyroid levels were slightly low.((this blood test was done at 15 4/7 weeks and at that time the baby was fine))What are the chances I'd miscarry this baby?? How concerned should i be about this low thyroid level? What should I be concerned about? Right now, I am a bundle of nerves. Thanks