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1. An investigation has been made into the effect on thyroxine secretion into bile of suddenly raising the thyroxine concentration in the liver.

2. Isolated rat livers, perfused with Tyrode solution, were injected with trace amounts of radioactive thyroxine. This injected hormone was captured by the binding sites in the tissue and then gradually secreted into the bile. A second injection of 100 µg stable (non-radioactive) thyroxine caused an enormous increase in the secretion rate. At its peak, the concentration of thyroxine in the bile was about 24 x its concentration in the liver (this was 10 x higher than the concentration ratio at tracer thyroxine levels). The effect extended well into the physiological range and injections of only 0·1 µg stable thyroxine caused a transient doubling of the secretion rate. Tri-iodothyronine also accelerated thyroxine secretion but it was 30 x less potent than thyroxine.

3. In contrast, the secretion of tri-iodothyronine into bile was only very slightly affected by injections of stable thyroxine or tri-iodothyronine, the maximum response being a 20-30% increase in the secretion rate.

4. This autoregulating mechanism would help to stabilize the concentration of thyroxine in the tissues but not the concentration of tri-iodothyronine.