THE PREDICTIVE VALUE OF SERUM THYROGLOBULIN IN THE FOLLOW-UP OF DIFFERENTIATED THYROID-CANCER

A strict and careful strategy has to be adopted to cure thyroid cancer . Diagnostic iodine-131 whole-body scan (WBS) and serum thyroglobulin (Tg) are important tools to detect thyroid remnants after thyroidectom y and radioiodine therapy. The aim of this retrospective study was to compare the relative sensitivity of WBS and Tg in the detection of thy roid remnants or metastases and to evaluate the predictive value of Tg in the clinical and scintigraphic course of the disease. Ninety-three patients were followed up after total thyroidectomy and the administr ation 4-6 weeks later of an ablative dose of 100 or 150 mCi I-131. Eig hty-five percent of the patients were free of regional or distant meta stases. The follow-up scheme included clinical examination of the pati ent followed by WBS, Tg, thyroid-stimulating hormone and free thyroxin e measurements performed 4 weeks after thyroxine withdrawal and the ob servance of a low-iodine diet for at least 1 week. WBS (+) patients re ceived a 100- or 150-mCi therapeutic dose of I-131. All patients were further followed up in the same way every 6 months until both WBS and Tg became negative, and thereafter at 1-, 2- and 4-year intervals. Six months after the postoperative radioiodine treatment (first visit), t he sensitivity of WBS and Tg was 87% and 26% respectively. Among patie nts who were WBS(+) at the first visit, 95% of those who were Tg(-) an d 47% of those who were Tg(+) had become disease-free at a median of 4 years after surgery (chi(2)=13.6; P<0.05). Patients whose tests were both positive required more radioiodine to be cured (335+/-90 vs 250+/ -95 mCi; P<0.05). Our data indicate that in early diagnosed thyroid ca ncer, serum Tg measured 6 months after the postoperative I-131 ablativ e dose is less sensitive than WBS for the demonstration of persistence of residual thyroid tissue but provides predictive information on the disease course. WBS(+) and Tg(-) patients are cured earlier and with less radioiodine than those who remain Tg(+).