The definition of a three-pool model for triiodothyronine (T3) distribution and metabolism in healty subjects by using vascular and extravascular sources

To evaluate T3 distribution, metabolism and excretion in normal subjects the authors used a three-pool model consisting of a plasma pool and a slow pool, whose basic approach was based on both vascular and extravascular sources. In particular plasma, stool and urine samples, and external counting over the liver, kidney and thigh were obtained following bolus injection of radiolabelled T3. Our results indicate that T3 can be considered as contained in a three-pool system where the plasma pool is interconnected with two extravascular composite tissue pools, i.e. a fast pool to represent mainly liver and kidneys and a slow pool to represent mainly skeletal muscle. In detail, the results indicate that: 1) the mean value for the total body size of T3 equals 35.25 μg; 2) the mean value for overall turnover rate of T3 equals 30.05 μg/day; 3) the mean amount of T3 leaving the system via fecal, urinary and metabolic losses is 14%, 5% and 81% of the overall turnover rate, respectively and 4) the mean amount of T3 metabolized by liver, kidneys and skeletal muscle is 27%, 2.5% and 28% of the overall turnover rate, respectively