Pathology of the autonomously functioning (hot) thyroid nodule.

We describe the pathologic findings of 73 clinically and scintigraphically confirmed hot nodules. In general, hot nodules from an unselected group primarily treated by surgery were smaller and the sex ratio was closer to equality compared with the ample female predominance in the referral, pre-, and post-prophylaxis groups. Malignancy was observed in six cases (8.2%) (5 follicular, 1 papillary). Of the 67 benign tumors, 48 (71.6%) were adenomas which showed the cytoarchitectural features of hot nodules described previously, and 19 (28.3%) were less well-differentiated adenomas that included a few oxyphil tumors. Intracolloid oxalate crystals from background thyroid tissue were present in 59 assessable cases (83%) overall, the majority showed more than occasional crystals that had a tendency to increase in number with decreasing morphologic activity of the thyroid epithelium. Thyroglobulin protein and mRNA stainings tended to be more pronounced in cell cytoplasm of the tumors than in background thyroid. This study shows that hot nodules may show a wide morphologic spectrum of follicular neoplasms and can be occasionally malignant. It is inferred from the morphologic and other findings that it is likely that some, if not all, of the primary follicular cancers associated with hyperfunction arise by clonal progression from benign hot nodules. This progression is rare, probably because most hot nodules present with the symptoms of hyperfunction and receive early treatment.