TSH levels outside the normal pregnancy range are associated with an increased risk of pregnancy complications as placental abruption, preterm birth, foetal death, and preterm breaking of waters (premature rupture of membranes). If you are taking thyroid medications then you should continue the treatment during pregnancy. Regular tests will guide you to dosage adjustments in pregnancy.
Low thyroid hormone is associated with adverse reproductive outcomes. The problems include miscarriage, pregnancy complications, and delayed foetal neurodevelopment. There is fair evidence that thyroid autoimmunity (positive thyroid antibody) is associated with miscarriage and infertility. Treating TSH levels >4.0 mIU/L is associated with improved pregnancy and miscarriage rates.