Factors influencing the early plasma disappearance rate and liver uptake of thyroxine

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.