Serum thyroglobulin measurements in thyroid cancer: evaluation of 'false' positive results

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 (greater than 5 micrograms/l) on initial measurement but no other evidence of residual, recurrent or metastatic cancer.

416 patients from several centres were followed for periods between 1 and 9 years. Serum Tg was measured at 6-12-month intervals.

All had differentiated thyroid cancer, treated by partial or total thyroidectomy and/or 131I ablation, and were receiving suppressive thyroxine therapy.

Serum Tg was measured and clinical, X-ray and scan assessment made of presence or absence of residual, recurrent or metastatic cancer.

Of 416 patients initially assessed, only 19 had Tg greater than 5 micrograms/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 131I with a subsequent fall in Tg; five had Tg between 5 and 20 micrograms/l with incompletely suppressed TSH levels; two subjects remained with slightly elevated Tg and undetectable TSH.

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.