Suppressed thyroglobulin performs better than stimulated thyroglobulin in defining an excellent response in patients...

According to the American Thyroid Association guidelines in 2015, both an unstimulated thyroglobulin (u-Tg) below 0.2 ng/ml and a stimulated thyroglobulin (s-Tg) below 1.0 ng/ml were required along with negative imaging findings to define an excellent response. This study aimed to investigate whether a u-Tg below 0.2 ng/ml coincides with a s-Tg below 1 ng/ml.

A total of 290 patients with nonmetastatic differentiated thyroid cancer were retrospectively evaluated with a median follow-up of 36 months. The levels of s-Tg were observed in patients whose u-Tg levels were below 0.2 ng/ml after radioiodine therapy, and risk factors associated with the increase of s-Tg to above 1 ng/ml from below 0.2 ng/ml were analyzed.

In total, 52.8% (153/290) of the patients achieved a u-Tg below 0.2 ng/ml 3 months after remnant ablation, most of whom (83.7%, 128/153) also achieved a s-Tg below 1 ng/ml. A total of 25 (16.3%) patients had an increased s-Tg above 1 ng/ml. A comparative analysis showed no significant difference between patients who showed an increase in thyroglobulin from below 0.2 ng/ml to above 1 ng/ml and those who did not. In a subgroup analysis assessing the influence of thyrotropin (thyroxin-stimulating hormone) on s-Tg, we enrolled 43 patients with at least two s-Tg measurements. We found that a higher level of thyroxin-stimulating hormone (118.23±30.72 vs. 59.99±26.12 µIU/ml) increased the s-Tg in 88.4% patients (P=0.00), which led to more patients (18.6-30.2%) with an increased s-Tg (to above 1 ng/ml) after thyroxin withdrawal.

Assessment of the level of u-Tg might be a better parameter to use for defining excellent response as u-Tg is more stable, convenient, economical, and is not associated with hypothyroidism as a side effect.