Background: Reverse T3 (rT3; 3,3',5'-triiodo-L-thyronine) is widely regarded as an inactive naturally occurring analog of thyroid hormone. rT3 is known to bind to the thyroid hormone analog receptor on plasma membrane integrin αvβ3. This integrin is generously expressed by tumor cells and is the initiation site for the stimulation by L-thyroxine (T4) at physiological free concentrations on cancer cell proliferation. Results: In the present studies, we show that rT3 caused increases of proliferation in vitro of 50% to 80% (P < 0.05–0.001) of human breast cancer and glioblastoma cells. Conclusion: rT3 may be a host factor supporting cancer growth.
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