Thyroid Gland - Ectopic Tissue

Image of ectopic tissue in the thyroid gland from a female F344/N rat in a subchronic study
Thyroid Gland - Ectopic tissue, Thymus in a female F344/N rat from a subchronic study. Well-formed thymic tissue is present adjacent to the thyroid gland.
Figure 1 of 2
Image of ectopic tissue in the thyroid gland from a female B6C3F1 mouse in a subchronic study
Thyroid Gland - Ectopic tissue, Thymus in a female B6C3F1 mouse from a subchronic study. Thymic tissue with cortex, medulla, and Hassall�s corpuscles is present within the thyroid gland parenchyma.
Figure 2 of 2
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comment:

Ectopic thymic tissue is a congenital change with the extrathymic tissue located either adjacent to ( Figure 1image opens in a pop-up window ) or within ( Figure 2image opens in a pop-up window ) the thyroid gland. The ectopic thymus may have both cortical and medullary tissue, as well as Hassall’s corpuscles. Ectopic thymus is a developmental abnormality that can occasionally be found in rats and mice and is generally unrelated to treatment, but exposures beginning during embryogenesis may potentially affect the occurrence of thymic ectopia.

recommendation:

Ectopic tissue in the thyroid gland should be diagnosed whenever present, but a severity grade is not needed for this change. The diagnosis should be modified with the tissue of origin (e.g., Thyroid gland - Ectopic tissue, thymus).

references:

Hardisty JF, Boorman GA. 1990. Thyroid gland. In: Pathology of the Fischer Rat: Reference and Atlas (Boorman GA, Eustis SL, Elwell MR, Montgomery CA, MacKenzie WF, eds). Academic Press, San Diego, 519–536.
Abstract: http://www.ncbi.nlm.nih.gov/nlmcatalog/9002563

Hardisty JF, Boorman GA. 1999. Thyroid and parathyroid glands. In: Pathology of the Mouse: Reference and Atlas (Maronpot RR, Boorman GA, Gaul BW, eds). Cache River Press, Vienna, IL, 537–554.
Abstract: http://www.cacheriverpress.com/books/pathmouse.htm