Thyroid Autoimmunity and Subclinical Hypothyroidism in Female Infertility: Recent Evidence, Clinical Controversies

Background: Thyroid dysfunction and thyroid autoimmunity are frequently identified during female infertility evaluation. Overt hypothyroidism clearly disrupts reproductive physiology, but the effects of subclinical hypothyroidism and euthyroid thyroid autoimmunity on natural conception and assisted reproductive outcomes remain controversial. Aim of this work is to review recent evidence on thyroid autoimmunity and subclinical hypothyroidism in female infertility. Methodology: This review was structured according to the Preferred Reporting Items for Systematic Reviews and Meta-Analyses 2020 framework. PubMed/MEDLINE, Embase, Scopus, Web of Science and the Cochrane Library were searched for studies published from January 2015 to July 2026. Randomized controlled trials, cohort studies, case control studies, systematic reviews, meta-analyses and professional guidelines evaluating thyroid autoimmunity, subclinical hypothyroidism, ovarian reserve, natural conception or assisted reproductive outcomes were considered. Evidence was synthesized narratively because of substantial variation in thyroid-stimulating hormone thresholds, antibody assays, infertility populations and reproductive outcomes. Results: Thyroid autoantibodies are present in approximately one-fifth of women evaluated for infertility. Observational studies suggest possible relationships between thyroid autoimmunity, diminished ovarian reserve, impaired embryo development and pregnancy loss, however, recent meta analyses and large cohort studies have not consistently demonstrated reductions in cumulative live birth. Subclinical or overt hypothyroidism, particularly when thyroid-stimulating hormone exceeds the assay-specific upper reference limit, appears more consistently associated with diminished ovarian reserve than isolated antibody positivity. Evidence supporting levothyroxine in women with thyroid-stimulating hormone concentrations of 2.5–4.0 mIU/L is also insufficient. Conclusion: Overt hypothyroidism should be treated before conception. Persistent subclinical hypothyroidism requires individualized assessment, with treatment becoming more reasonable as thyroid-stimulating hormone rises above approximately 4–6 mIU/L. Euthyroid thyroid autoimmunity should prompt biochemical monitoring rather than routine levothyroxine treatment. IAHS Medical Journal Vol 9(1), June 2026;107-113