Possible explanations for patients with discordant findings of serum thyroglobulin and 131I wholebody 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 131I remnant abla-tion and thyroid hormone suppression of thyroid-stimulating hormone (TSH). Sensitive surveillance for DTC recurrence and metastases includes radioiodine diagnostic whole-body scan-ning (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 cor-related with the results of DWBS. In general, undetectable Tg levels with negative DWBS (DWBS) suggest complete remis-sion, whereas detectable or elevated Tg concentrations are suggestive of the presence of 131I uptake in local or distant metastases. However, DTC patients with discordant results of Tg measurement and 131I WBS have been observed in the follow-up study. Negative 131I DWBS and a positive Tg test (DWBS Tg) are found in most of these cases. Positive 131I DWBS and a negative Tg test (DWBS Tg), though of uncom-mon 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 131I WBS or serum Tg. Explanations for the discordance are speculative but should be scrutinized when confronted with discrepant data in a given patient. Key Words: differentiated thyroid carcinoma; thyroglobulin; 131I; whole-body scannin