An examination of the extent of reversibility of thyroxine binding within the thyroxine distribution space in the rabbit

1. New Zealand white rabbits were injected intraperitoneally with [¹²⁵] L‐thyroxine. Experiments were performed after 24 hr had elapsed. 2. Thyroxine distribution space was calculated as 129 ml. kg⁻¹. Half‐life of disappearance of thyroxine from the blood of the urethane anaesthetized rabbit was 653 min. 3. After exchange of approximately 20 ml. kg⁻¹ whole blood with heparinized blood from a non‐radioactive donor, equilibration of radioactivity in the recipient had produced a peak radioactivity by 2 hr. 4. The thyroxine space in which complete exchange took place within 2 hr was 102·7 ± 4·7 ml. kg⁻¹ in six rabbits. This is 79·6% of the thyroxine distribution space. 5. These results are compared with observations in the rat.