Factors Influencing the Early Plasma Disappearance Rate and Liver Uptake of Thyroxine

Abstract

The early disappearance from plasma (T 1/2) of i.v. administered thyroxine (T₄) labelled with ¹³¹I was studied in order to find a simple biochemical parameter with which it could be correlated in health and disease. The radioactivity of the liver was also measured by surface counting to determine the time for the peak liver count rate. The T 1/2 for the ¹³¹I–T₄ showed a correlation of borderline significance with T₄ in plasma but a good correlation was found with T₄-binding globulin (TBG) in plasma and an even better with the T₃-test. The T₃-test showed a significant correlation with TBG but when the T₄-binding capacity of prealbumin (TBPA) was taken into account the correlation was even better indicating that the result of the T₃-test was not only dependent on the TBG concentration in plasma but also on the TBPA.

The T 1/2 correlated best with the time for the liver peak count rate. The study supports the hypothesis that in the early distribution phase of T₄, the free binding sites extracellularly and the available intracellular binding locations compete for T₄ until a state of equilibrium is reached.