Triiodothyronine accelerates maturation of bile acid metabolism in infant baboons

We tested the hypothesis that triiodothyronine (T₃) treatment accelerates the early postnatal maturation of bile acid metabolism in the baboon. Infant baboons were implanted with 21-day-release pellets containing T₃ (n = 12), a placebo pellet (n = 6), or no pellet (n = 13). T₃ treatment increased plasma T₃ concentrations from 3.0 to 5.0 nmol/l between birth and 15 wk of age. At 15 wk of age, bile acid pool sizes, fractional turnover rates (FTR), and synthetic rates were determined by an isotope-dilution method with ³H- and ¹⁴C-labeled cholic (CA) and chenodeoxycholic acid (CDCA). T₃ treatment increased CA pool size by 47% and CA synthetic rate by 37% but did not significantly affect CDCA pool size or synthetic rate. Consequently CA-to- CDCA pool size ratio (0.77 vs. 0.42) and biliary CA-to-CDCA concentration ratio (0.88 vs. 0.46) were higher in the T₃-treated infants than in combined placebo-treated and non-treated control infants. T₃ treatment did not affect the bile acid glycine-to-taurine conjugate ratio, CA FTR, or CDCA pool size, FTR, and synthetic rate. T₃ treatment lowered plasma high-density lipoprotein fraction 2 and 3 cholesterol concentrations by 22 and 40%, respectively. T₃ treatment also increased hepatic low-density lipoprotein receptor mRNA levels but did not affect plasma low-density lipoprotein cholesterol concentrations. We conclude that modest elevation of plasma T₃ during the preweaning period increases the CA-to-CDCA ratio at the end of the preweaning period to near adult values.