Incidence of neoplasia in Hashimoto's thyroiditis: a fine-needle aspiration study.

There is a recognized association between Hashimoto's thyroiditis (HT) and thyroid neoplasms. We reviewed fine-needle aspirations (FNAs) from 90 patients with HT to assess the contribution of this procedure. For seven patients, FNA showed HT and follicular neoplasm (n = 6) or HT and papillary carcinoma (n = 1). Eighteen patients underwent thyroid resection. Three patients had follicular adenomas which were not detected by FNA, one patient had papillary carcinoma confirmed, and six patients with follicular neoplasm by FNA were negative for tumor. Thus, 4% of our patients had confirmed neoplasms, an incidence lower than usually reported. One reason for the lower rate of neoplasia in our series was misinterpretation of follicular neoplasia in the background of HT. The cytologic changes in the hyperplastic follicular and metaplastic oncocytic epithelium are similar to those seen in follicular neoplasm. Our study suggests that these processes may be indistinguishable, and thus, in the presence of HT, the diagnosis of follicular neoplasm probably should not be rendered.