Possible explanations for patients with discordant findings of serum thyroglobulin and 131I whole-body scanning

The long-term monitoring of patients with differentiated thyroid carcinoma (DTC) is essential throughout the patient's life after total or near-total thyroidectomy followed by I-131 remnant ablation and thyroid hormone suppression of thyroid-stimulating hormone (TSH). Sensitive surveillance for DTC recurrence and metastases includes radioiodine diagnostic whole-body scanning (DWBS) and measurement of serum thyroglobulin (Tg) after endogenous or exogenous TSH stimulation. Serum Tg levels during thyroid hormone withdrawal (Tg-off) are usually well correlated with the results of DWBS. In general, undetectable Tg levels with negative DWBS (DWBS-) suggest complete remission, whereas detectable or elevated Tg concentrations are suggestive of the presence of I-131 uptake in local or distant metastases. However, DTC patients with discordant results of Tg measurement and I-131 WBS have been observed in the follow-up study. Negative I-131 DWBS and a positive Tg test (DWBS- Tg(+)) are found in most of these cases. Positive I-131 DWBS and a negative Tg test (DWBS+ Tg(-)), though of uncommon occurrence, has also been demonstrated in a small but significant number of cases. With this scenario, one should first attempt to uncover a cause for possibly false-negative or false-positive I-131 WBS or serum Tg. Explanations for the discordance are speculative but should be scrutinized when confronted with discrepant data in a given patient.