[Value of the assay of thyroglobulin for postoperative surveillance of differentiated thyroid cancers]

Thyroglobulin has been evaluated among 30 control subjects and 81 thyroidectomized patients with differentiated thyroid carcinoma (50 papillary, 15 follicular, 16 mixed). 40 presented without residual thyroid tissue in the neck, 27 with residual tissue, 14 with metastases. Tg evaluation was performed on (78 dosages) and off thyroid hormone therapy (74 dosages), before and after withdrawal of thyroid medication in 25 patients. Tg was measured at different periods after surgical or radioiodine therapy of metastases (7 cases). The sera containing anti-Tg antibodies are rejected. The mean Tg levels was 11.9 +/- 8.5 ng/ml in the control group. In the group of patients with thyroid cancer, Tg levels were dependent on several factors: presence or not of residual thyroid tissue, presence or absence of a replacement therapy. All the patients on or off thyroid medication with metastases except one, presented with plasma Tg levels alone 5 ng/ml. In conclusion, plasma Tg appears as a good index for the research of metastases but only in patients without anti-Tg antibodies and residual thyroid tissue in the neck. Despite the existence of false negative results, a Tg undetectable in treated patients presenting a normal roentgen chest leads to avoid total body radioiodine scan and other usual radiologic investigations. On a opposite hand a plasma Tg greater than 50 ng/ml is highly suggestive of the presence of metastases.