Relation between Estrogen Receptor and Malignancy of Thyroid Cancer

The relationship between the histological grade of dedifferentiation of thyroid cancer and estrogen receptors (ER) was examined immunohistochemically. Thyroid cancers were from postmenopausal females of almost the same mean age (69‐73 years old) and within the same period of time (1974–1983). ER immunoreactivity located in the nucleus of the epithelium was found in all 6 well differentiated papillary cancers, and 5 of them (83.3%) showed ER‐immunoreactive (ER‐IR) cells amounting to 20 or more per visual field (x 100) under a light microscope. Of the 6 cases of poorly differentiated papillary cancer, 5 (83.3%) had 1‐19 ER‐IR cells per visual field. ER‐IR cells were negative in 5 out of 6 cases (83.3%) of anaplastic cancers. Thus, the number of ER‐IR cells tended to decrease with the degree of atypism of thyroid cancer (P < 0.001).