Answer: Non-neoplastic thyroid tissue with exrathyroidal extension

Histology: The interface between the thyroid parenchyma and the soft tissues surrounding the thyroid is blurred in these sections showing the absence of an unambiguous thyroid capsule. Thyroid follicles are seen permeating the perithyroidal soft tissues with frank extension into skeletal muscle. Importantly, the cells lining the follicles do not exhibit and appreciable atypia, and the follicles do not induce a stromal reaction.

Discussion: Lack of anatomic integrity of the thyroid capsule can sometimes cause confusion between extrathyroidal extension of normal and neoplastic thyroid tissue. The notion that the thyroid gland is completely invested by an anatomic thyroid capsule is erroneous. At the microscopic level, this thin fibrous capsule is incomplete such that thyroid follicles are often noted within perithyroidal soft tissues (including skeletal muscle!).

Before interpreting extrathyroidal extension as unequivocal evidence of invasive tumor growth, keep in mind that 90% of thyroids demonstrate extension of follicles into peri-thyroidal connective tissue, and 7% show thyroid tissue within the skeletal muscle. The latter finding is usually noted in the midline strap muscle associated with the pyramidal lobe. Unlike thyroid carcinoma, this non-neoplastic extra-thyroidal extension does not elicit a host fibrotic response.