Thyroglobulin assay in the follow-up of patients with differentiated thyroid carcinomas: comparison of its value in patients with or without normal residual tissue.

The usefulness of serum thyroglobulin (Tg) assay in the follow-up of differentiated thyroid carcinomas has been evaluated in 109 subjects divided into two groups. Group 1 included 64 patients who had undergone total thyroid ablation. In 40 of the 41 patients in complete remission serum Tg was undetectable during replacement therapy (TSH below 5 microunits/ml). In 18 out of the 40 patients serum TG was detectable following endogenous TSH stimulation. As 83% of these patients had ectopic uptake prior to the last radioiodine treatment, this release of Tg under TSH stimulation suggests the persistence of occult neoplastic tissue. Of the other 23 patients, 20 had bone or lung metastases and 3 patients had lymph node recurrences: in all these patients, serum Tg was detectable during replacement therapy and increased after TSH stimulation. Group 2 included 45 patients in whom normal residual thyroid tissue was present at the time of the investigation. Of these, 35 patients were in apparent remission and 19 of them had detectable Tg level within the normal range. The other 10 patients had detectable metastases and in 4 of these the Tg level was also within the normal range. Thus, no conclusion can be drawn from a normal Tg level in the presence of residual thyroid tissue. Bovine TSH stimulation did not improve significantly the diagnostic value of Tg assay in this group of patients.