Case 1: Sheets of fibrosis devoid of normal thyroid parenchyma.

In some areas, islands of residual thyroid epithelium remain, surrounded by heavy inflammation.

Higher power demonstrates strands of follicular epithelium surrounded by lymphocytes, plasma cells and rare eosinophils.

Yet another residual island of atrophic follicular epithelium with a heavy inflammatory infiltrate.

Dense fibrosis is a key finding.

not a great image, supposed to show residual muscle cells, but may need to delete Image

Riedel thyroiditis is a rare autoimmune inflammatory disorder. It is characterized by fibrosis that replaces the thyroid parenchyma and extends past the thyroid capsule. Often, the fibrotic process causes the thyroid gland to adhere to adjacent structures, eliciting concern for a malignant process. Riedel thyroiditis may be part of IgG4-related systemic disease.

The differential diagnosis includes the fibrosing variant of Hashimoto thyroiditis, however, the fibrosing process in Hashimoto does NOT extend past the thyroid capsule.

Most patients are actually euthyroid, but up to 1/3 may have hypothyroidism. Complications may occur due to extension beyond the thyroid and adherence to local structures i.e. dysphasia, airway obstruction.