Levothyroxine Intervention in Pregnant Women with TSH （2.5 MIU/L-upper Limit of Reference Range） and Negative Thyroid Peroxidase Antibody

"Participants in our study will determine the dosage of L-T4 based on their weight (BW):~1. BW ≥50kg: Starting with a daily dose of 1 tablet;~2. BW \< 50kg: Starting with a daily dose of half a tablet.~At 12 weeks, 18-20 weeks, 24-26 weeks, and 34-36 weeks of gestation, the serum TSH, FT3, and FT4 will be measured, and investigator will adjust dosage according to TSH:~1. When TSH \> ULRR, the participants will receive L-T4 in addition to their original medication, and additional unplanned follow-up;~2. When TSH at 2.5mIU/L-ULRR, add 1/4 tablet of medictaion;~3. When TSH at LLRR -2.5mIU/L, maintain the original dose;~4. When TSH \<LLRR, reduce 1/4 tablets of medictaion.~5. If TSH continues to be lower than LLRR after discontinuation, investigator will measure FT4, TRAb and other indicators to determine if it is clinical hyperthyroidism. If so, antithyroid drugs will administered according to guidelines. Participants will stop all trial medictaion after delivery."

"Participants in our study will determine the dosage of placebo based on their weight (BW):~1. BW ≥50kg: Starting with a daily dose of 1 tablet;~2. BW \< 50kg: Starting with a daily dose of half a tablet.~At 12 weeks, 18-20 weeks, 24-26 weeks, and 34-36 weeks of gestation, the serum TSH, FT3, and FT4 will be measured, and investigator will adjust dosage according to TSH:~1. When TSH \> ULRR, the participants will receive L-T4 in addition to their original medication, and additional unplanned follow-up;~2. When TSH at 2.5mIU/L-ULRR, add 1/4 tablet of medictaion;~3. When TSH at LLRR -2.5mIU/L, maintain the original dose;~4. When TSH \<LLRR, reduce 1/4 tablets of medictaion.~5. If TSH continues to be lower than LLRR after discontinuation, investigator will measure FT4, TRAb and other indicators to determine if it is clinical hyperthyroidism. If so, antithyroid drugs will administered according to guidelines. Participants will stop all trial medictaion after delivery."