L-triiodothyronine at a slightly over physiologic dose increases leucine flux, which suggests an increase in protein degradation in normal subjects

The present study was undertaken to determine whether minor alterations of serum thyroid hormone concentration have an effect on plasma amino acid kinetics. Flux rates of 2H3-labeled leucine, 15N-labeled leucine, and 2H3-labeled alanine were determined in the basal state and after a 7-day course of L-triiodothyronine (T3) supplementation in seven normal adult subjects. A small dose of T3 was given (0.8 microgram/kg/day) that resulted in only slightly higher than physiologic serum T3 concentration. After T3 treatment, leucine carbon flux increased from 1.22 +/- 0.05 to 1.40 +/- 0.05 mumol/kg/minute (p less than 0.001) and leucine nitrogen flux increased from 2.10 +/- 0.15 to 2.48 +/- 0.14 mumol/kg/minute (p less than 0.005), but alanine carbon flux did not change significantly, going from 6.26 +/- 0.67 to 6.58 +/- 0.58 (p = not significant). Because the experiment was performed in the postabsorptive state, increased leucine flux after T3 treatment suggests accelerated protein degradation