Serum thyroglobulin (Tg) measurements for posttreatment monitoring of differentiated thyroid carcinoma (DTC), is a key component of recurrence detection,1 in particular for patients with near total or total thyroidectomy, with or without radioiodine ablation. The negative predictive value of a very low or undetectable posttreatment serum Tg measurement is reported as 98.6%2; however, for those patients that experience recurrence, sensitivity for detection of recurrent DTC is only around 80%, even with, highly sensitive serum Tg assays.3 In fact, there are reports of metastatic DTC recurrence in the presence of undetectable serum Tg.4, 5, 6, 7, 8 It remains unclear whether the lack of detectable serum Tg in these cases is due to assay interference from the presence of anti-Tg antibodies (TgAbs), a lack of Tg production by the recurrent tumor, insufficient sensitivity of current Tg assays, or other factors.

One of the limitations of most current Tg immunoassays (Tg-IAs) is potential analytical interference by TgAbs. TgAbs are detectable in approximately 20% to 30% of patients with thyroid cancer, while less than 10% of the general population have detectable levels of these antibodies.9, 10, 11 TgAbs can cause either a false increase or false decrease in Tg-IAs, depending on the format.11,12 Immunometric assays, which comprise most commercial Tg-IA utilized in current clinical practice (including the Beckman Access Thyroglobulin IA discussed in this report), exhibit a false decrease in Tg measurement in the presence of TgAbs, potentially yielding false-negative results when monitoring for disease recurrence. Therefore, it is recommended that TgAb always be measured in conjunction with Tg-IA measurement to assess the reliability of the Tg measurement.1 In order to overcome TgAbs interference with Tg-IA, mass spectrometry (MS)–based assays incorporating trypsin digestion of all sample proteins (including TgAbs), followed by immuno-enrichment of Tg-specific peptides and detection by liquid chromatography (LC)-MS/MS methodology, have been developed. While early versions of these Tg mass spectrometry (Tg-MS) assays lacked sufficient analytical sensitivity for non-thyroid stimulating hormone–stimulated Tg monitoring postthyroidectomy,13 current iterations of Tg-MS have enhanced analytical sensitivity comparable to high sensitivity Tg-IA methods (0.1-0.2 ng/mL). This is considered sufficient for non-thyroid stimulating hormone–stimulated surveillance and has been shown to effectively identify the presence of Tg in some patients with TgAbs and undetectable Tg by Tg-IA.12 Since Tg-IA remains the most sensitive method for patients without TgAbs, testing algorithms starting with assessment of TgAbs status, followed by Tg-IA measurements if TgAb-negative and Tg-MS if TgAb-positive are commonly used for posttreatment surveillance.14,15

Measurement of Tg in fine needle aspiration biopsy (FNAB) washout fluid is also a well-established and useful method for investigating potential recurrence of DTC in suspicious lymph nodes (LNs) or nodules arising from the thyroid bed in postthyroidectomy patients. FNAB washout Tg has been demonstrated to be much more sensitive than cytology alone,16, 17, 18, 19 with a reported sensitivity of greater than 90%. The combination of FNAB cytology and FNAB washout Tg can achieve greater than 95% sensitivity and 90% specificity for recurrent metastatic DTC.16

Despite advancements in Tg testing modalities, there are still patients with metastatic DTC to LNs, as confirmed by FNAB cytology, which appear to not have detectable serum Tg. This observation raises the question of whether these patients remain serum Tg-negative or undetectable due to (1) absence of clinically significant Tg production; (2) TgAbs interfering with Tg detection; or (3) nonsecretory Tg production. The aim of this study was to determine, if in patients with metastatic DTC to LNs and undetectable Tg in serum, Tg could be detected in the FNAB washout fluid, and if so, whether an explanation for the lack of detectable Tg in serum can be elucidated.