A model of thyroxine metabolism based on the effects of environmental temperature.

1. Evidence is presented that, in rats, the whole body and plasma half-lives of [(131)I]thyroxine are different.2. It is shown that the whole body half-life can be influenced by environmental temperature: plasma half-life is influenced to a much lesser extent.3. It is demonstrated that, in the rabbit, free labelled thyroxine and plasma bound labelled thyroxine leave the plasma at different rates when administered by intravenous injection.4. It is deduced from the above and other evidence that the currently accepted view of thyroxine distribution and metabolism is invalid and a model compatible with the evidence is presented and discussed.5. The essential feature of the model is that protein binding in plasma or in various tissues, once it has occurred, may be regarded as being irreversible and that no significant interchange takes place.6. Indices of thyroxine metabolism are derived. These enable thyroxine content and turnover of the whole body or of individual organs to be measured.