Nodular disease and parafollicular C-cell distribution: results from a prospective and retrospective clinico-pathological study on the thyroid isthmus

Objective: The isthmus represents a peculiar, as yet partially unexplored, thyroid gland area. Aim of the study: To assess i) the prevalence and clinico-pathological features of solitary thyroid isthmic nodules (STIN); ii) the frequency of medullary thyroid carcinoma (MTC) arising from the isthmus; and iii) the C-cell distribution in the isthmus of patients with MTC and benign nodular thyroid disease (NTD). Subjects and methods: Patients referred from 2006 to 2008 for STIN were prospectively recruited, and underwent serum calcitonin (C-t) measurement and fine needle aspiration cytology (FNAC). MTCs diagnosed from 1993 to 2005 were retrospectively searched. Immunohistochemistry was performed using anti-C-t antibodies on lateral lobes and isthmi of 50 benign NTD and 50 MTC cases. Results: From 1993 to 2005, 150 patients underwent surgery for MTC. All patients had the neoplasm located in lateral thyroid lobes, none in the isthmus. In the 3 years following, 192 STIN patients (40 (21%) males, 152 (79%) females; mean age: 46.2 +/- 7.1 years; 6.4% of NTD subjects) were recruited. All had normal Ct concentrations. FNAC was malignant or suspicious for malignancy in 14 (7.3%) patients. Histology found malignancy in 17 (9%) cases, MTC in none. C cells were disclosed in lateral thyroid lobes of 100% MTC and 77% benign NTD patients; isthmi were free of C cells in either group. Conclusions: STINs are significantly less likely to be MTC in patients presenting with sporadic disease. Therefore, Ct screening is not warranted in these subjects. Nonetheless, STINs are more likely to be neoplastic and deserve equal attention as those of the lateral lobes.