Riedel's Thyroiditis is an uncommon form of chronic thyroiditis characterized by an invasive fibrosclerosis of the gland, often involving surrounding tissue. The relationship of Riedel's Thyroiditis to other forms of thyroiditis is not clear. We presented a 47 year-old woman first diagnosed with sub-acute thyroiditis based on clinical findings and laboratory results. Eight months later, she had a thyroidectomy operation due to an enlargement of the thyroid gland and symptoms of compression. Histopathologic evaluation showed that she had Riedel's Thyroiditis, but there were some histopathologic findings of sub-acute thyroiditis as well. Until now, there has only been one case reported in which Riedel's Thyroiditis was diagnosed in a patient with a history of sub-acute thyroiditis in the literature. Although aetiology of Riedel's Thyroiditis is unknown, it may develop in the course of sub-acute thyroiditis.