Vitamin D3 is commonly used as a supportive therapy in Hashimoto's Disease, an autoimmune form of hypothyroidism. The scientific rationale stems from multiple observational studies showing that vitamin D deficiency is more prevalent among individuals with autoimmune thyroid diseases, including Hashimoto’s. Vitamin D is known to have immunomodulatory effects—it influences the activity of T lymphocytes and cytokine production, both of which play roles in autoimmunity. Some interventional studies suggest that supplementation with vitamin D3 can reduce anti-thyroid antibody levels (such as anti-TPO antibodies) in patients with Hashimoto's, potentially slowing disease progression or reducing inflammation.

However, the evidence remains moderate (hence the rating of 3). While several small randomized controlled trials and meta-analyses indicate improvements in immune markers, robust data linking vitamin D3 supplementation directly to improved clinical outcomes (such as thyroid hormone levels or symptom reduction) are still lacking. Most major endocrinology societies recommend screening for and correcting vitamin D deficiency in patients with autoimmune thyroid disease, but do not recommend high-dose vitamin D3 solely for Hashimoto’s unless there is a documented deficiency.

In summary, vitamin D3 is used based on scientific evidence of immunological benefit and high rates of deficiency in this population, but its role is primarily as a supportive therapy rather than a stand-alone treatment.