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To collect and assess clinical reports of a putative relationship between thyroid state and the biology of cancers of various types.A number of prospective case–control studies reviewed here have suggested that subclinical hyperthyroidism increases risk of certain solid tumors and that spontaneous hypothyroidism may delay onset or reduce aggressiveness of cancers. Small case studies have reached similar conclusions. A controlled prospective trial of induced hypothyroidism beneficially affected the course of glioblastoma. A context in which to interpret such findings is the recent description of a plasma membrane receptor for thyroid hormone on cancer cells and dividing tumor-associated endothelial cells.Accumulating clinical evidence may justify new, broadly-based controlled studies in cancer patients of the possible contribution of thyroid hormone to tumor behavior.