The guidelines are summarized as follows: 1. Subclinical hypothyroidism is defined as TSH exceeding the upper bound of normal (4.5-5.0 mIU/L), while FT4 is normal. 2. The reference range of TSH during pregnancy is highly variable, with the upper boundary of normal value for non-pregnant women in the vast majority of laboratories being 4 mIU/L and 2.5 mIU/L in early pregnancy. 3. Whether to apply the upper boundary of TSH in early pregnancy (e.g. > 2.5 mIU/L) as a cut-off value for the diagnosis and treatment of subclinical hypothyroidism in women attempting pregnancy remains controversial, which is the reason for the development of this guideline. 4. There is insufficient evidence to suggest that subclinical hypothyroidism (defined as TSH > 2.5 mIU/L but normal FT4) is associated with infertility. 5. There is good evidence that subclinical hypothyroidism (defined as TSH > 4 mIU/L but normal FT4) is associated with miscarriage, but there is insufficient evidence that TSH levels between 2.5-4 mIU/L are associated with miscarriage. 6. There is good evidence that treatment of subclinical hypothyroidism can improve pregnancy rate and reduce miscarriage rate when TSH >4 mIU/L. 7. There is good evidence that subclinical hypothyroidism with TSH > 4 mIU/L during pregnancy is associated with poor developmental outcome, but randomized studies have found that treatment does not improve developmental outcome. 8. There is good evidence that thyroid autoimmunity is associated with miscarriage and infertility. Treatment with levothyroxine may improve pregnancy outcomes in thyroid-positive patients, especially in cases with TSH levels > 2.5 mIU/L. 9. There is good evidence against recommending extensive screening for thyroid function during pregnancy. However, screening is recommended for women with high-risk factors (family or personal history of thyroid disease, physical examination or symptoms suggestive of goiter or hypothyroidism, type 1 diabetes, infertility, history of miscarriage or preterm delivery, personal history of autoimmune disease). Specific recommendations include: 1. The available data support the rationale for testing TSH in infertile women attempting pregnancy. If TSH exceeds the non-pregnancy laboratory reference range (usually > 4 mIU/L), patients should be treated with levothyroxine to keep TSH below 2.5 mIU/L (level B evidence). 2. Due to limited information, if TSH levels prior to pregnancy are between 2.5-4 mIU/L, treatment options include monitoring TSH levels and treating if TSH > 4 mIU/L or treating with levothyroxine to keep TSH < 2.5 detectable (level C evidence). 3. Treatment is recommended if TSH >2.5 mIU/L in early pregnancy (level B evidence). 4. Although thyroid antibody testing is not routinely recommended, testing for anti-thyroid peroxidase (TPO) antibodies may be considered when TSH is > 2.5 mIU/L on repeated tests or when other risk factors for thyroid disease are present (level C evidence). 5. If anti-TPO antibodies are present, TSH levels should be monitored and treatment should be considered when TSH > 2.5 mIU/L (Level B evidence).