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In Riedel thyroiditis, the gland is often asymmetrically involved. The affected portions become rock hard and cut with great difficulty. Dense fibrous bands may extend from the thyroid capsule to involve the neck muscles and other nearby structures. Thyroid parenchyma is replaced by hyalinized fibrous tissue. Inflammatory infiltrate can be quite prominent and usually consists of plasma cells (mostly IgG-4 producing), lymphocytes, and macrophages. Numerous eosinophils may also be seen. The main differential diagnosis of Riedel thyroiditis is fibrosing variant of Hashimoto thyroiditis. In Hashimoto thyroiditis, fibrosis is limited to the thyroid and there is extensive oxyphilic change in the follicular epithelium.