Thyroxine-like Actions of 3′-Isopropyl-3,5-dibromo-L-thyronine, a Potent Iodine-Free Analog

The biological potency of an iodine-free thyroxine analog, 3′-isopropyl-3,5- dibromo-L-thyronine, is about 7 times that of L-thyroxine (T4) when estimated from its effect on the resting heart rate of thyroidectomized rats but only about twice that of T4 when estimated from its metamorphosis-accelerating actionaction in Rana catesbeiana tadpoles or its goiterpreventing action in rats. Although the corresponding diiodo analog shows generally greater potency, the considerable activity of the bromo compound indicates that iodine is not specifically required for thyroxine-like biological effects. (Endocrinology 80: 1143, 1967)