The activation of the innate and the adaptive immune responses contributes to the control and eradication of tumors. Natural killer cells and T lymphocytes have a crucial role in tumor immunosurveillance, and their migration to the tumor site is of significant prognostic value for cancer patients. However, these cells are not the only protagonists in the tumor microenvironment (TME) scenario. Other cellular components, like macrophages, dendritic cells, neutrophils, and cancer-associated fibroblast, as well as cytokines and hormones, also shape tumor growth and spreading. Among the last ones, thyroid hormones emerged as essential actors, being responsible not only for the tumor cell growth but also for the modulation of the immune cell activity in the TME. We here describe the direct regulatory effects of thyroid hormones on cancer cell proliferation and their central actions upon cells of the immune system that would, in turn, affect tumor growth and dissemination. The potential consequences of this relationship on cancer therapy are also discussed.