Metastatic medullary and undifferentiated thyroid cancers are not associated with elevated levels of serum thyroglobulin.

It is now well accepted that serum thyroglobulin (Tg) determination is a specific marker of disease activity in well-differentiated thyroid cancer after total thyroidectomy. In this study we examined 15 thyroidectomized patients with metastases from medullary thyroid cancer and 10 patients with metastases from undifferentiated thyroid cancer, to determine whether serum Tg measurement could be useful also in these types of thyroid tumors. In contrast with differentiated thyroid cancer, which had high levels of circulating serum Tg, patients with medullary and undifferentiated thyroid cancer had undetectable or low levels of serum Tg. Thus, serum Tg measurements are not useful in the diagnosis of metastatic disease in these patients.