Tissue eosinophilia and eosinophil degranulation in Riedel's invasive fibrous thyroiditis.

The etiology of Riedel's invasive fibrous thyroiditis (IFT) has
remained obscure. This rare disorder has been confused in the past
with the more common fibrous variant of Hashimoto's disease. The
typical histological features of IFT, in particular the presence of
an invasive fibrosclerotic process in conjunction with a prominent
chronic inflammatory infiltrate, suggest that the release of
fibrogenic cytokines and other factors from these cellular
infiltrates may play an important role in the pathogenesis of this
condition. Our observations in routinely processed tissue sections
obtained from patients with documented IFT of striking tissue
eosinophilia led us to hypothesize that eosinophils and their
products may play a role in the evolution of this disease.
Immunofluorescence staining with affinity-purified polyclonal
rabbit antibody directed against human eosinophil granule major
basic protein revealed marked tissue eosinophilia and abundant
extracellular deposition of major basic protein in all specimens
from 16 patients with IFT. By contrast, only occasional eosinophils
and no extracellular major basic protein were detected in control
thyroid tissues obtained from patients with multinodular goiter,
Graves' disease, Hashimoto's disease, and normal thyroid tissue.
The presence of marked eosinophil infiltration and extracellular
major basic protein deposition in IFT and other associated
fibrosclerotic conditions suggests a role for eosinophils and their
products in propagating the fibrogenesis seen in IFT.