Blood-brain transport of triiodothyronine is reduced in aged rats.

An age-related decline in blood-brain barrier transport of thyroid hormones may contribute to the central nervous system changes with aging. To test this hypothesis, the brain uptake index (BUI) of levo (L) and dextro (D) triiodothyronine (T3) was determined in male Fischer 344 rats at 6 months of age (young) and 26 months of age (aged). Young rats pair fed with aged were included to control for reduced food intake in aged rats. The L-T3 BUI of aged rats (22.4 +/- 2.1%) was significantly reduced compared to young rats (29.5 +/- 2.0%) or young rats pair fed with aged rats (28.5 +/- 2.5%) (p less than 0.05). This could not be attributed to age-related changes in BBB permeability or to reduced cerebral blood flow. At steady state conditions, the brain uptake of either L-T3 or D-T3 was not altered with aging. There were no significant changes in plasma or brain binding of T3. These results indicate that the reduced BBB transport of T3 in aged rats is counterbalanced by a reduction in T3 clearance from the brain.