SERUM THYROGLOBULIN MEASUREMENTS IN THYROID-CANCER - EVALUATION OF FALSE POSITIVE RESULTS

OBJECTIVE Serum thyroglobulin (Tg) should be undetectable in patients successfully treated for thyroid carcinoma. We have examined the course of disease in 19 patients with raised serum Tg (> 5-mu-g/l) on initial measurement but no other evidence of residual, recurrent or metastatic cancer. DESIGN 416 patients from several centres were followed for periods between 1 and 9 years. Serum Tg was measured at 6-12-month intervals. PATIENTS All had differentiated thyroid cancer, treated by partial or total thyroidectomy and/or I-131 ablation, and were receiving suppressive thyroxine therapy. MEASUREMENT Serum Tg was measured and clinical, X-ray and scan assessment made of presence or absence of residual, recurrent or metastatic cancer. RESULTS Of 416 patients initially assessed, only 19 had Tg > 5-mu-g/l but no clinical or radiological evidence of disease. At follow-up, 11 patients had developed overt signs of malignancy; one had been treated with I-131 with a subsequent fall in Tg; five had Tg between 5 and 20-mu-g/l with incompletely suppressed TSH levels; two subjects remained with slightly elevated Tg and undetectable TSH. CONCLUSION Patients with elevated Tg require careful follow-up even in the apparent absence of disease. Moderate elevation of serum Tg may be due to inadequate thyroxine suppression therapy, assessed by detectable TSH values measured in a sensitive assay.