Thyroxine and Triiodothyronine Kinetics and Extrathyroidal Peripheral Conversion Rate of Thyroxine to Triiodothyronine in Healthy Elderly Humans

Summary

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 > 0.05) could be found in any of the kinetic parameters of T₄ between the elderly and young groups. However, the following significant (p < 0.05) changes were noted for T₃: an increased disposition constant in the phase of slow distribution (β), shortening of the half-life of this phase (T 1/2β), 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 > 0.05) were observed for the percent turnover of T₄ converted to T₃.

Key-Words:

Kinetics - Elderly - Thyroid Hormones