A DISCUSSION OF THE ISOTOPE DILUTION METHOD FOR ESTIMATING SECRETORY RATES FROM URINARY METABOLITES

Abstract

The formula for estimating the secretory rate of a hormone after the intravenous administration of a single injection of a radioactive tracer of the hormone and isolation of one of its urinary metabolites [image]has been reexamined considering a two compartment model. sa.t In this scheme the tracer is postulated to be injected into the same compartment into which the hormone is secreted. Mathematical analysis has shown that, just as in a single compartment model, the formula is valid provided the assumptions made in its derivation are maintained. However, certain situations are conceivable to which this formula would not apply. One of these is pregnancy, in which hormone made by the placenta is secreted into two compartments simultaneously (maternal and foetal pools) and metabolism occurs differently in each compartment. Secretory rate determinations of oestrogens in pregnant women by the isotope dilution method appear to be subject to the limitations defined by the model presented. Experimental data have been presented which may be rationalized by such a consideration.