Antioxidant activity of thyroxine and related substances. Effect on in vitro cell growth

• 1.1. As anticipated from the postulated hormone, antioxidant relationship, the thyroid hormones and their precursors have elicited significant growth responses of human cells in vitro.
• 2.2. Growth was enhanced by 10⁻⁶ M and 10⁻⁷ M concentrations of thyroxine and triodothyronine, recognized in the human system as circulating, (transport) and intracellular forms of thyroid hormone respectively.
• 3.3. 10⁻⁶ M and 10⁻⁷ M concentrations of diiodotyrosine, a non-hormone in the whole animal but an immediate precursor of thyroxine, also stimulated growth of the cell cultures.
• 4.4. Diiodothyronine, thyronine and tyrosine, in these concentrations especially the latter acted as growth inhibitors. These compounds are not only weaker antioxidants than the active hormone but they are also readily autooxidizable, forming toxic quinones.
• 5.5. The effects of the growth enhancing compounds, include “adaptation efficiency” as well as cell multiplication. The forming following our definition, is judged to be of importance as an index of the physiologic state and “acclimatization” of cells in culture systems.