Higher thyroid hormone levels and cancer.

Many studies, from in-vitro and in-vivo to population-based studies, demonstrate undesirable effects of higher thyroid hormone levels on the development of cancer, as well as its prognosis and consequently an outcome of cancer patients. Thyroid hormones mediate cancer cells' growth, proliferation, and metastatic diffusion. Current data demonstrate an increased risk of solid as well as hematologic malignancies in patients with higher serum thyroid hormone levels and/or lower thyrotropin levels (TSH). However, the results are sparse. The present chapter will summarize studies investigating the correlation between hyperthyroidism, higher upper reference range triiodothyronine (T3) and thyroxine (T4) levels, and normal-low TSH with cancer risk and prognosis, respectively. The diagnostics, clinical management, and treatment of hyperthyroidism in cancer patients will also be illustrated. Finally, the importance of a prompt restoration of euthyroidism in cancer patients and the priority role of radioiodine (RAI) therapy to permanently eliminate hyperthyroidism in most cancer patients will be discussed.