Abstract
Chronic autoimmune thyroiditis (CAT), a common autoimmune thyroid disorder, is the leading cause of hypothyroidism in iodine-sufficient regions and is characterized by thyroid autoimmunity, chronic inflammation, and progressive structural thyroid changes. Although levothyroxine (LT4) restores biochemical euthyroidism, it does not directly address the underlying autoimmune process, highlighting the need for adjunctive therapeutic strategies. Photobiomodulation (PBM), also known as low-level laser therapy (LLLT), has been proposed as a non-invasive intervention with potential immunomodulatory and tissue-level effects. A systematic narrative review was conducted following the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) reporting principles. PubMed/MEDLINE, Google Scholar, and additional databases were searched for original human clinical studies evaluating PBM/LLLT in CAT, including studies using the term Hashimoto's thyroiditis (HT), and reporting thyroid-related outcomes. Due to heterogeneity in study designs and PBM protocols, findings were synthesized narratively. Six eligible clinical studies published between 2010 and 2025 were identified. Across studies, PBM was associated with reductions in thyroid autoantibodies, improvements in thyroid hormone indices, and decreases in LT4 dose requirements. Longer follow-up studies reported ultrasonographic changes, while one sham-controlled trial demonstrated improvements in oxidative stress markers and quality of life (QoL) without short-term endocrine changes. However, current evidence is limited by the small number of human studies, heterogeneous PBM protocols, and the frequent use of concomitant interventions such as selenium or vitamin D. Overall, PBM may represent a promising adjunctive approach in CAT, although randomized sham-controlled trials are required before clinical implementation.