[What diagnosis can be made by the pathologist in a case of sclerosing thyroiditis?]

The pathology of thyroiditis seems well-established with a recognized classification based on clinical and pathological features. However, problems of differential diagnosis remain between both Riedel's and Quervain's thyroiditis and sclerosing Hashimoto's thyroiditis: these entities sometimes lack the characteristic histological pattern, and the clinico-biological data are not always available to the pathologist. We re-examined 18 cases of thyroiditis with sclerosis, retrieved from our files, diagnosed as Riedel's thyroiditis in five cases, Quervain's thyroiditis in five other cases and sclerosing Hashimoto thyroiditis in eight cases. Only two diagnosed cases of Riedel's thyroiditis were pathognomic. Three cases of Quervain's thyroiditis and four cases of sclerosing Hashimoto's thyroiditis presented a slight or moderate extension of the fibrosis in perithyroidal soft-tissues, raising the differential diagnosis of an incipient Riedel's thyroiditis. A definite diagnosis of the type of thyroiditis with sclerosis remains difficult, because all three pathologies present common points. In cases with a characteristic pattern, the diagnosis is straightforward. However, it appears in our study that half of the diagnoses remain ambiguous, because of the existence of histological features common to different entities. In these cases, we think the diagnosis of sclerosing thyroiditis NOS would be more appropriate, the histology not being sufficiently characteristic to make a more specific diagnosis.