In the present investigation we studied serum anti-thyroglobulin and anti-thyroid microsomal autoantibodies, measured by hemagglutination technique, in 600 patients with thyroid cancer seen by us from 1975 to 1985 (mean follow-up 46 months). Positive thyroglobulin antibodies and/or microsomal antibodies were found in 138 (23%) patients (23.9% with papillary, 25% with follicular, 16.1% with anaplastic, and 4.1% with medullary thyroid carcinomas). The incidence of positive tests was similar in each decade of life (ranging between 21.9% and 27.9%), whereas in a normal sex-matched population with no evidence of thyroid disease, the frequency of positive tests was very low in young people and increased to 23% in people older than 60. In 64 patients with no evidence of residual or metastasic thyroid tissue after surgery and radioiodine, initially positive antibody titres became negative in 54.6%, decreased in 32.8%, did not change in 3.1%, and increased in 9.3%. On the contrary, antibody titres of patients with persistent disease became undetectable in 8.3%, decreased in 16.6%, remained unchanged in 25%, and increased in 50%. The clinical course of differentiated thyroid cancer was unaffected by the presence of thyroid antibodies and no difference was found in the death rate between antibody-positive and antibody-negative patients (11.5% and 13.6%, respectively). In conclusion, our data indicate that: 1) autoimmune phenomena are not an infrequent finding in thyroid cancer; 2) as in non-malignant thyroid diseases, positive-antibody tests are more frequently observed in females than in males; 3) at variance with normal controls, no age-dependent increase in serum anti-thyroid antibodies was found in thyroid cancer; 4) the presence of metastatic thyroid tissue seems to be necessary to perpetuate the autoantibody synthesis, and 5) anti-thyroid autoantibodies are not a protective or worsening factor in the tumour outcome.The relationship between thyroid autoimmunity and thyroid cancer is still unclear. No evidence of increased incidence of antithyroid-microsomal autoantibodies was found by Kornstad (1974) in a large series of thyroid cancer patients using the complement fixation technique. In a few similar studies using the more sensitive passive hemagglutination method, a significant increase in the incidence of anti-thyroglobulin and microsomal autoantibodies has been reported in thyroid ma¬ lignancies, with positivity ranging from 2.3 to 15% for thyroglobulin antibodies and from 9.1 to 21% for microsomal antibodies (Kornstad 1974;Amino et al. 1975;DeGroot et al. 1976;Pinchera et al. 1977;Mariotti et al. 1978). The main limita¬ tion of most of the above investigations was the rather small number of patients studied and the lack of adequate follow-up.In this study we re-evaluated the incidence of thyroglobulin antibodies and microsomal anti¬ bodies in a large series of patients with thyroid cancer. We also correlated the behaviour of serum antibodies with several clinical parameters and with the tumour outcome.

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