A study published in the Hellenic Journal of Nuclear Medicine showed that vitamin D supplementation lowered thyroid antibodies in 218 Greek patients with Hashimoto's thyroiditis and normal TSH levels. Hashimoto's was diagnosed on the basis of elevated thyroid peroxidase (TPO) and/or thyroglobulin antibodies and ultrasound results  Because these antibodies are also seen in patients with Graves' disease and subacute/postpartum thyroiditis a number of these patients may have had other thyroid conditions.

Despite being exposed to adequate sunlight, 85.3 percent of the patients had vitamin D insufficiency/deficiency based on 25 OH Vitamin D levels less than 30 ng/ml.  Patients with the lowest levels of vitamin D had the highest levels of TPO antibodies.  Supplementation with 1,200 to 4,000 IU vitamin D3 daily by the deficient group for four months resulted in a 20.3% decrease in TPO antibodies by the end of the study.

Resource:  "Vitamin D Supplementation Shows Promise for Hashimoto's Thyroiditis." 2015. Hellenic Journal of Nucear Medicine. Sept-Dec; 18(3): 222-7.