Autoimmune Thyroiditis
In focal thyroiditis the thyroid is usually normal in size and contains foci of lymphocytes which are predominantly T cells, although lymphoid follicles can also occur. Thyroid cells adjacent to these foci are usually atrophic and deficient in colloid, but away from the foci, thyroid follicular architecture is normal. Focal thyroiditis may also be prominent adjacent to a papillary carcinoma or other neoplasm. By contrast, the whole thyroid is usually involved in Hashimoto’s thyroiditis. The lymphocytic infiltrate is more ex tensive, diffuse, and composed mainly of T cells, with prominent germinal centres containing B cells scattered through the gland (Figure 1). Macrophages, dendritic cells, and sometimes giant cells may be prominent. The thyroid follicles suffer variable degrees of destruction, depending largely on chronicity, and in the process undergo hyperplasia and oxyphil metaplasia, giving rise to so- called Hürthle or Askanazy cells. These cells are generally absent in juvenile autoimmune thyroiditis.
Fig1. Histological features of (a) normal thyroid, (b) atrophic thyroiditis, and (c) Hashimoto’s thyroiditis. Original magnification ×100; photomicrographs courtesy of Dr K. Suvarna.
However, the relative proportion of lymphocytic infiltrate, thy roid follicular cell change, and fibrosis varies greatly, in keeping with the suggestion made previously that there is a broad spectrum of changes which may ultimately result in atrophic thyroiditis. In this condition, the thyroid is small, has extensive fibrosis mixed with a scattered lymphocytic infiltrate, and there is a marked reduction in thyroid follicular cells. A distinct subset of patients with Hashimoto thyroiditis has been delineated recently with high circulating IgG4 levels and IgG4- positive plasma cells in the thyroid, accompanied by extensive fibrosis, and a high frequency of hypothyroidism. The pathology in postpartum and silent thyroiditis generally resembles mild to moderate Hashimoto’s thyroiditis, although without the oxyphil metaplasia. Germinal centres are usually absent.
Graves’ Disease
It is now unusual to see the full histological picture of Graves’ disease as patients are almost all treated with antithyroid drugs which diminish the lymphocytic infiltrate.( 4) Even after such treatment, however, there is often a diffuse or focal lymphocytic thyroiditis, predominantly of T cells, sometimes with germinal centre formation. As an aside, lymphoid hyperplasia may also involve the lymph nodes, thymus, and spleen in Graves’ disease, once again being reversed by antithyroid drugs. The thyroid follicles are both hypertrophied and hyperplastic, with scalloping and reduction in colloid (Figure 32). The epithelial cells are columnar and extend as papillae into the lumen. These changes are also attenuated by antithyroid drugs, so that after prolonged treatment, the colloid re- accumulates, the papillae regress, and the epithelium becomes cuboidal.
Fig2. Histological features of Graves’ disease. Original magnification ×100; photomicrographs courtesy of Dr K. Suvarna.
