The goal of this clinical trial is to learn if levothyroxine (L-T4) works to treat pregnant women with TSH 2.5 mIU/L-the upper limit of reference range (ULRR) of pregnancy and TPOAb-negative. It will also learn about the safety of L-T4. The main quesitons the investigator want to answer are:

• Will L-T4 reduce miscarriage rates and have an impact on pregnancy complications in pregnant participants?
• What medical issues do participants have when taking L-T4 during pregnancy? -Investigators will compare L-T4 with placebo (a substance with a similar appearance without medication) to see if L-T4 could reduce miscarriage rates

Participants should:

• Take L-T4 or placebo during the whole pregnancy.
• Visit the hospital once every 6-8 weeks during pregnancy for checkups and tests
• Keep a diary of their pregncny complications and daily record of L-T4 or placebo intake.
• Visit the hospital for examination 42 days postpartum for checkups and follow up by phone at 6 and 12 months postpartum.

Inclusion Criteria:

1. Women of childbearing age（18-45 years old）, natural pregnancy, singleton pregnancy.
2. Measure thyroid related indexes within 8 weeks of pregnancy: TSH 2.5mIU/L-ULRR, FT4 is normal and TPOAb negativity.
3. Willing to sign an informed consent form.

Exclusion Criteria:

1. History of recurrent miscarriage (≥3 times).
2. Assisted reproduction (artificial insemination, in vitro fertilization and embryo transfer);
3. Suffer from diseases that seriously affect pregnancy outcome, including hypertension, diabetes, heart disease, liver and kidney dysfunction, etc.
4. Failure of vital organs.
5. Except autoimmune thyroid disease，suffer from other autoimmune diseases.
6. Thyroid diseases (including hyperthyroidism, thyroid cancer, thyroid amyloidosis and other extensive intrathyroidal diseases, current subacute thyroiditis, iodine-deficiency endemic goiter, thyroidectomy or 131I treatment, previous thyroid Ultrasound prompts diffuse thyroid disease, etc.).
7. Use of thyroid-related drugs (lithium carbonate, thioureas, sulfonamides, sodium para-amino salicylate, potassium perchlorate, phenylbutazone, sulfate, tyrosine kinase inhibitor, etc.) during screening and affect thyroid Functional testing drugs. (Including glucocorticoids, metoclopramide, propranolol, amiodarone, sodium valproate, etc.)
8. Secondary hypothyroidism or central hypothyroidism. (Including pituitary tumors, surgery, radiotherapy, lymphocytic hypophysitis, etc.)
9. L-T4 allergy.
10. Unwilling to sign an informed consent.
11. Other clinicians judged that they are not suitable to participate in the study.

the clinical information of the participants and the primary outcome data of the LIGHT study will be shared

IPD will be available half year after the last participants finished the study