Unstimulated Highly Sensitive Thyroglobulin <0.2 ng/mL: Insufficient to Predict Stimulated Thyroglobulin <1 ng/mL?

Thyroglobulin (Tg) measurement is a cornerstone of follow-up in patients with differentiated thyroid cancer, as emphasized by both the Korean Thyroid Association and American Thyroid Association guidelines [1,2]. Over the past three decades, Tg assays have advanced through three generations. Typically, the limit of detection of one generation corresponds to the functional sensitivity (FS) or limit of quantification (LOQ) of the next, resulting in a 5- to 10-fold improvement in analytical sensitivity [3-5], as shown in Table 1.

Table 1.

Assay Generation-Specific Limits of Detection and Functional Sensitivity (or Limit of Quantification) for Thyroglobulin

Assay generation	Limit of detection	Functional sensitivity or limit of quantification
First (conventional)	0.2 ng/mL	0.9 ng/mL
Second (highly sensitive)	0.035–0.1 ng/mL	0.15–0.2 ng/mL
Third (ultrasensitive)	0.01 ng/mL	0.06 ng/mL

The most widely accepted standard for long-term surveillance is a stimulated Tg level of 1 ng/mL, measured at least 6 months after total thyroidectomy and radioiodine ablation, when normal thyroid tissue has been eradicated. This cutoff lies above the FS of first-generation Tg assays and has been validated over decades of follow-up. Tg levels are highly dependent on thyroid-stimulating hormone (TSH) stimulation. Depending on the amount of residual thyroid tissue or disease burden, stimulated Tg values may rise approximately 5- to 10-fold compared with unstimulated levels. Thus, TSH stimulation was required to overcome the limited sensitivity of earlier assays. However, TSH stimulation involves inherent disadvantages: it can be accomplished through either thyroid hormone withdrawal, leading to symptomatic hypothyroidism, or administration of recombinant human TSH, which is costly. Because stimulated Tg values are typically 5 to 10 times higher than unstimulated values, second-generation assays with FS or LOQ of 0.15 to 0.2 ng/mL theoretically enable unstimulated measurements to replace the traditional 1 ng/mL stimulated cutoff. Such assays were developed more than two decades ago and have since been validated in multiple studies [4].

With advances in assay technology, third-generation platforms have achieved FS or LOQ as low as 0.06 ng/mL. Yet, their clinical utility remains uncertain. In this issue, Jang et al. [5] evaluated a third-generation assay in 268 patients who had undergone total thyroidectomy for differentiated thyroid carcinoma. Serum samples obtained before TSH stimulation were measured simultaneously using second- and third-generation assays and compared with stimulated Tg values measured using a second-generation assay. With cutoffs of 0.105 ng/mL (second-generation) and 0.12 ng/mL (third-generation), the third-generation assay demonstrated higher sensitivity (72.0% vs. 39.8%) but lower specificity (67.2% vs. 91.5%) than the second-generation assay. These findings underscore two key points. First, since Tg serves as a surveillance marker, higher sensitivity is clinically valuable. Some patients classified as having stimulated Tg ≥1 ng/mL may be missed if only unstimulated second-generation assays are used, whereas third-generation assays may mitigate this limitation. Second, current Korean and American guidelines classify an unstimulated Tg level of <0.2 ng/mL as an ‘excellent response’ for patients who have undergone total thyroidectomy and/or neck dissection followed by radioactive iodine therapy [1,2]. This threshold likely reflects the FS or LOQ of second-generation assays (0.15 to 0.2 ng/mL), on which most prior studies were based. As more evidence is collected using third-generation assays, it may become possible to redefine the threshold for an ‘excellent response’ as unstimulated ultrasensitive Tg <0.1 ng/mL in patients who have undergone total thyroidectomy and radioiodine ablation.

Nonetheless, even ultrasensitive third-generation assays have clinical limitations. In patients who underwent lobectomy or did not receive radioiodine ablation after total thyroidectomy, Tg elevation may be caused by residual normal thyroid tissue rather than recurrence. Additionally, anti-Tg antibodies can interfere with measurements, resulting in falsely low or inconsistent values. In such cases, long-term trends in Tg, along with complementary imaging—such as a diagnostic radioiodine whole body scan, ultrasonography, computed tomography, or positron emission tomography—remain indispensable. Future developments should focus on assays that detect Tg isoforms secreted exclusively by cancer cells or that mitigate interference from endogenous anti-Tg antibodies.