Simple Summary: The usefulness of calcitonin measurement in fine needle aspiration (FNA-CT) for the diagnosis of medullary thyroid carcinoma (MTC) has been confirmed by several authors. During the review process of records of our patients with thyroid nodules (TNs) and elevated serum CT, we focused our attention on 11 patients with TNs showing at histology thyroid neoplasms other than MTC, as compared with a MTC patients' group. We showed that a high FNA-CT concentration is not only found in MTC but may also be observed in benign/malignant TNs coexisting with diffuse C-cells hyperplasia (CCH) due to contamination of the needle through the CCH area. Since diffuse CCH may cause false positive FNA-CT results, we propose a cautious use of FNA-CT in patients with TNs with increased serum CT and high FNA-CT values to avoid unnecessary surgery. Purpose: The FNA-CT is useful for the diagnosis of MTC. The aim of this study was to evaluate the performance of FNA-CT in TNs coexisting with CCH. Methods: This study retrospectively reviewed the records of 11 patients with TNs submitted to thyroidectomy on the basis of elevated basal and/or stimulated serum CT values, which at histology were not confirmed to be MTC. The results obtained in this group were compared with those of a previously reported group of histologically proven MTC patients submitted to an identical presurgical evaluation. All patients, negative for known mutations in the RET proto-oncogene, were preoperatively submitted to neck ultrasound, FNA-cytology, and FNA-CT. Results: Approximately 6 of 11 patients showed increased (>36 ng/mL, as established in previous studies not involving patients with CCH) FNA-CT. All these patients showed diffuse CCH at histology in the thyroid lobe submitted to FNA; 5 of them were benign at histology, while only one was malignant (papillary thyroid carcinoma, PTC). The remaining 5 of 11 patients had low FNA-CT (<36 ng/mL), and all of them showed only focal CCH in the lobe submitted to FNA; three of them were malignant (2 PTC, 1 follicular carcinoma), while two were benign. Conclusions: Employing the currently proposed cut-off values, false-positive FNA-CT results may be observed in benign/malignant TNs with coexisting diffuse CCH. FNA-CT must therefore be cautiously used in the diagnostic approach for patients with TNs and a slightly increased basal or stimulated serum CT concentration in order to avoid unnecessary surgery. [ABSTRACT FROM AUTHOR]