The influence of age on the kinetics of thyroxine (T4) and triiodothyronine (T3), and on peripheral conversion of T4 to T3, has been studied, comparing a group of euthyroid, healthy, elderly individuals to a group of young individuals. A monocompartment model was adopted for the kinetics of T4, while a bicompartment model was used for T3. No significant difference (p greater than 0.05) could be found in any of the kinetic parameters of T4 between the elderly and young groups. However, the following significant (p less than 0.05) changes were noted for T3: an increased disposition constant in the phase of slow distribution (beta), shortening of the half-life of this phase (T 1/2 beta ), elevation of the central distribution volume Vc and the elimination constant K23, reduction of the K12: K23 ratio, and increased plasma metabolic clearance and hormonal daily turnover. No significant differences (p greater than 0.05) were observed for the percent turnover of T4 converted to T3.

The kinetics of triiodothyronine (T3) and thyroxine (T4) were studied in 20 ewes. Ten were used as the control group, the other 10 received massive hepatic lesions induced by the ingestion of carbon tetrachloride. Half of each group were i.v. injected with 125 IT3 or 125 IT4, following a plasma disappearance over 96 hours. The amount of hormone present in each plasma sample was determined by immunoextraction procedure. The data was adjusted to the open two-compartment modelThe hepatic insufficiency provoked a decrease in the disposition constant of T3, an increase in the plasma half-life of the distribution phase as well as a moderately decreased turnover rate of this hormone.The T4 kinetics were altered during acute hepatic insufficiency in the decrease of the elimination constant K13 and hormone plasma turnover rate.The study of the peripheral conversion rate based on the kinetic data showed an extrathyroidal conversion of T4 to T3 in the control sheep, none being detected in the sheep with induced hepatic insufficiency.