Influence of a single meal on fractional disappearance and catabolic rates of 3,5,3'-triiodothyronine and thyroxine over 24 hours.

The influence of a single meal on the rates of catabolism of 3,5,3'-triiodothyronine (T3) and thyroxine (T4) was investigated in young pigs. Plasma hormone concentrations, fractional disappearance rates (K), distribution volumes and catabolic rates were estimated during three periods after a meal. For T3, plasma concentration and K were greatest immediately after the meal and decreased progressively. Catabolic rate decreased from 0.45 nmol.hr-1.kg-1 immediately after the meal to 0.28 nmol.hr-1.kg-1 after 24 hr. In a separate investigation, a meal was found to cause an increase in plasma [125I] T4, indicating a shift in the distribution of the hormone pool. Catabolic rate of T4 appeared to be greatest in the period immediately after the meal and decreased to 0.43 nmol.hr-1.kg-1 nearly 24 hr later.