Medicine General b09b3996 Most common cause of Thyroiditis is: A Reidl 's Thyroiditis B Subacute Thyroditis C Hashimoto's Thyroiditis D Viral Thyroiditis High-Yield Explanation Answer is C (Hashimoto's Thyroiditis) Hashimotas thyroiditis, also called chronic lymphocytic thyroiditis, is the most common firm of thyroiditis.