Use of thyroxine- and triiodothyronine-specific antibodies to study thyroxine kinetics in rainbow trout, Salmo gairdneri

The paper deals with the evaluation of thea priori possibility of estimating the unknown parameters of a system having a known structure via an input-output experiment (structural identifiability). The problem is analysed with reference to biological compartmental systems (and precisely to a specific but largely representative class of these systems, i.e. the so-called strongly connected compartment systems). The analysis is developed by using concepts and methods of system theory and particularly the concepts of ‘controllability’ and observability (i.e. the possibility of influencing the behaviour of the whole system by the input and of estimating it from the output). Two new theorems are proved concerning the necessary and sufficient conditions for the observability and controllability of strongly connected compartmental systems. On the basis of these theorems, a structural identifiability criterion is established and a digital-computer implementation technique for this criterion is given. Some typical tracer-analysis experiments on biological compartmental systems are analysed with the aim of evaluating how many and which parameters of the considered model can be estimated through the chosen input-output experiments.

Structural identifiability of strongly connected biological compartmental systems.

1. Evidence is presented that, in rats, the whole body and plasma half-lives of [131I]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.

https://physoc.onlinelibrary.wiley.com/doi/pdf/10.1113/jphysiol.1969.sp008694

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

The turnover of thyroxine in the plasma of the domestic fowl (Gallus domesticus).

A model is proposed to describe the early thyroxine distribution in man. The four main compartments, plasma, liver, interstitial fluids and other tissues, are characterized by their transfer rate constants and by their distribution spaces.

A E Dumont

Papers

Studies in iodine metabolism. 3. three-pool systems of extrathyroidal thyroxine kinetics.