Carbohydrate feeding increases total body and specific tissue 3,5,3'-triiodothyronine neogenesis in the rat

The glucose-fed rat, in contrast to the chow-fed animal, has a higher serum total T3 concentration and an increase in the hepatic content of T4 5'-deiodinase (type I) activity. The mechanism and significance of these glucose-induced changes in T3 metabolism are elucidated in this study. To focus on extrathyroidal thyroid hormone metabolism the kinetic parameters were determined in thyroidectomized T4-replaced rats (1.25 micrograms T4/100 g BW.day). Kinetics of T4 and T3 were studied separately by infusing labeled hormone to equilibrium. Glucose feeding for 72 h (G) significantly increased both the total and free serum T3 concentrations compared to the respective means in the chow-fed control group (P). The glucose-induced changes in serum T3 reflect the approximate doubling of T3 production to 14.7 +/- 0.6 ng/h.100 g in G rats compared to 7.6 +/- 0.7 ng/h.100 g in P rats. The higher T3 production rate in the G group is due to a significant increase in the fractional total body T4 to T3 conversion (0.33 +/- 0.02) compared to that in the P group (0.19 +/- 0.02). The tissue (liver, kidney, brain, and brown adipose tissue) concentration of T4 (nanograms per g wet wt) was significantly increased in the G group. The increase ranged from 54% in liver to 80% in kidney, brain, and brown adipose tissue. The tissue concentration of T3 (nanograms per g wet wt) was even more dramatically increased by glucose feeding than was T4. The glucose-induced increment in organ T3 ranged from 2.5-fold (kidney, muscle, and brain) to 5-fold (liver and white adipose tissue) to 12-fold (brown adipose tissue). These data indicate that the increase in serum total and free T3 concentrations associated with glucose feeding reflects augmented total body T3 production from T4. The effect of the enhanced T3 neogenesis was generalized, as the T3 content was increased in each organ studied. Thus, glucose feeding has unique effects on T3 metabolism.