Objective: Serum thyroglobulin (Tg) is the marker of differentiated thyroid carcinoma (DTC) after total thyroidectomy, but its value is limited by the interference of anti-Tg antibodies (TgAb). Detection of Tg in fine-needle aspiration biopsy (Tg-FNAB) washout fluid is used to identify neck DTC recurrences/metastases, but the interference of serum TgAb in this procedure is unknown.
Patients and methods: Seventy-three patients (41 after surgery for thyroid cancer and 32 with thyroid nodules) evaluated for suspicious cervical lymph nodes were retrospectively reviewed. Tg was assayed by immunoradiometric assay or chemiluminescent assay in ultrasound-guided FNAB used for cytology. Serum TgAb were detected by passive agglutination or chemiluminescent assay. On the basis of preliminary data obtained in lymphadenitis, Tg-FNAB more than 36 ng/ml and more than 1.7 ng/ml (in the presence or absence of thyroid gland, respectively) was considered as indicative of metastasis.
Results: In 51 TgAb-negative patients, Tg-FNAB was positive in 15 (12 with malignant and three with nondiagnostic cytology), all with histologically confirmed DTC metastases. Of the remaining 36 patients with negative Tg-FNAB, 30 had nonsuspicious and six had suspicious cytology. Histology of the latter showed four undifferentiated thyroid cancer metastases and two lymphadenitis. In 22 TgAb-positive patients, Tg-FNAB was positive in 14 (12 with malignant and two with nondiagnostic cytology), all with histologically confirmed DTC metastases.
Conclusions: Clinical performance of Tg-FNAB appears to be not substantially affected by TgAb, and this procedure remains superior to cytology in the identification of DTC neck metastases. However, cytology should always be performed because, irrespective of TgAb, Tg is undetectable in FNAB from undifferentiated metastases.