Study of the kinetics of the thyroid hormones and the peripheral conversion rate of thyroxine to triiodothyronine in acute renal insufficiency under experimental conditions.

We studied the kinetic behavior of triiodothyronine (T3) and thyroxine (T4) in two groups of 10 sheep. One was used as the control group (I) and the other consisted of sheep with acute renal failure (A.R.F.) induced by an iv. injection of mercuric dichloride. Five sheep of each group were injected with 125IT3 and the remaining five with 125IT4, later taking periodical extractions throughout the first 96 hours. The radioactive contaminants in the plasma were eliminated by extracting the hormones through immunoextraction. The mathematical calculation of the kinetics was based on the open two-compartment model. The A.R.F. caused a decrease in the disposition constant of the slow phase in the T3 kinetics, while at the same time prolonging the plasma half-life of the phase. The steady state distribution volume increases and the distribution constant K21 decreases in the group suffering from renal insufficiency. As regards the T4, all the kinetic parameters were altered with the exception of the distribution volumes, the most significant changes being the shortening of the plasma half-life in the end phases, the rise of the elimination constant K13 and the increase in the plasma clearance and thyroxine turnover rates. The analysis of the peripheral conversion rate revealed with a great amount of variation, that there is evidence of the conversion of T4 to T3 in the healthy group while this phenomenon remains undetected in the sick animals.