Hashimoto's thyroiditis (HT) is the most commonly observed inflammatory and autoimmune disease of the thyroid gland. HT is the main cause of spontaneous hypothyroidism in adults. Symptoms of hypothyroidism may develop within a few years in patients with initial euthyroidism. Many genetic and environmental factors play a role in the pathogenesis of the disease, including iodine exposure, drugs, chemicals, toxins, infections and smoking. In recent years, the relationship between oxidative stress level and thyroid autoantibodies in HT has attracted increasing attention of researchers. In the studies, it has been reported that oxidative stress levels may increase due to chronic inflammation, insufficient thyroid hormone levels, excessive autoimmune response and excessive iodine intake. OS occurs as a result of an imbalance between free radical production and antioxidant defense mechanisms. Data from clinical studies clearly show that the balance between oxidants and antioxidants shifts towards the oxidative side in patients with autoimmune thyroiditis, suggesting that oxidative stress may be a key event in the pathophysiology of the disease, independent of thyroid function. Gerenova et al. divided patients with a diagnosis of HT into three subgroups according to thyroid function (group 1-euthyroid, group 2-hypothyroid, and group 3-Levothyroxin treated patients) and pointed out cellular antioxidative deficiency in all stages of the disease. Standard treatment in patients with HT is lifelong levothyroxine (LT4) treatment, which adjusts the dose to reach normal circulating thyrotropin (TSH) levels. Studies have found persistent fatigue, psychological and general well-being, and impairments in cognitive function in patients with HT despite LT4 replacement therapy. Gomez et al. compared 152 hypothyroid women treated with levothyroxine and 238 women without hypothyroidism disorder. Female patients with hypothyroidism, despite being euthyroid, showed worse quality of life than the control group. It was emphasized that the quality of life of women with hypothyroidism should be evaluated and further research should be conducted. Recent evidence has suggested that low-level laser therapy (LLLT) can improve thyroid function and reduce levels of thyroid peroxidase antibodies (TPOAb) in patients with hypothyroidism caused by chronic autoimmune thyroiditis. LLLT is a non-invasive, painless, low-risk and low-cost method that uses the interaction of light with molecular structures to support anti-inflammatory effects and regeneration of biological tissues. In the literature, data examining the effects of LLLT on oxidative stress level and quality of life in patients with HT is limited. As far as we know, it will be the first study examining the effect of LLLT on oxidative stress, fatigue and quality of life in cases diagnosed with HT. The planned research is trying to improve and expand the existing literature on LLLT in cases with HT and to contribute to the treatment protocol in order to reduce the negative consequences that may be experienced in this group of patients now and in the future. These statements show the originality of the study. Purpose: The aim of the study is to examine the effects of LT4 treatment combined with LLLT on thyroid autoimmunity, oxidative stress, fatigue and quality of life in patients with Hashimoto's diagnosis.