There are various guidelines for pregnancy and thyroid disease as different countries have a different set of guidelines. Some are more up to date than others:
2012 – Management of Thyroid Dysfunction during Pregnancy and Postpartum: An Endocrine Society Clinical Practice Guideline
https://ign.org/cm_data/2012_deGroot_Management_of_thyroid_dysfunction_during _pregnancy_and_postpartum_JCEM.pdf
2014 – 2014 European Thyroid Association Guidelines for the Management of Subclinical Hypothyroidism in Pregnancy and in Children
https://www.karger.com/Article/Fulltext/362597
2014 – Guidelines for the Treatment of Hypothyroidism
https://www.liebertpub.com/doi/pdfplus/10.1089/thy.2014.0028
2015 – Management of primary hypothyroidism: statement by the British Thyroid Association Executive Committee
https://www.british-thyroid-association.org/sandbox/bta2016/bta_statement_on_the_management_of_primary_hypothyroidism.pdf
2017 – Guidelines of the American Thyroid Association for the diagnosis and management of thyroid disease during pregnancy and the postpartum
https://www.liebertpub.com/doi/full/10.1089/thy.2016.0457?url_ver=Z39.88-2003&rfr_id=ori:rid:crossref.org&rfr_dat=cr_pub%3dpubmed
2017 -Thyroid hormone treatment among pregnant women with subclinical hypothyroidism: US national assessment
https://www.bmj.com/content/356/bmj.i6865