Abstract
Plasma levels of T3 and T4 were measured in male and female Fischer 344 (F344/Cr1) inbred rats aged 10, 17, 30, 56, 80 and 106 weeks in order to document the hormone profile of this strain of rat. In general, levels of T4 were higher in males than in females. Levels of T3 were lower in females for the first 17 weeks, similar for rats of age 30 weeks, and thereafter higher in the females than the males. T4 concentration decline was age-associated in both sexes. T3 levels declined in the males only, but remained relatively constant in the females throughout their lifespan. Generally, T4/T3 ratios declined throughout the lifespan of the Fischer rat.