The paper presents experimental data on extrathyroidal metabolism of thyroid hormones in rats of various ages (1.5–32 months): T4-131I distribution in an organism, indices of T4 and T3 kinetics, tissue deiodinative capacity, formation of T4 tissue metabolism products and their clearance from cells and organism, T4 binding with tissue proteins. Essential changes in T4 peripheral tissue metabolism have been found with aging: T4 binding by tissues is decreased; T4 distribution space and metabolic clearance are increased; 125I clearance with urine is decreased; T4 half-time in blood remains unchanged; tissue deiodinative capacity is enhanced. Indices of T3 peripheral metabolism in old rats remain unchanged. All these findings result in shifts of the T4/T3 ratio in favour of the latter and in invariability of its content in tissues, alongside with a decrease of functional activity of the thyroid gland with agin