A theoretical five-pool model to evaluate triiodothyronine distribution and metabolism in healthy subjects

We have examined the in vivo production, distribution, metabolism, and excretion of radiolabeled triiodothyronine (T3) in eight normal humans using a new five-pool mammillary model which includes four extravascular pools to represent liver, kidney, skeletal muscle, and all other unquantified tissues. Trace amounts of [125I]T3 and [131I]T3 were injected and plasma, stool and urine samples and external counting of liver, kidney, and thigh were drawn following bolus injection of radiolabeled T3. Our results indicate that the skeletal muscle pool represents the largest pool in our system, being 42% of the total-body pool size of the hormone (Qtot = 0.52 +/- 10% [CV] micrograms/kg body weight [BW]). The plasma pool QA (0.090 +/- 9% [CV] micrograms/kg BW) contains approximately 17% of Qtot, while the size of the unquantified tissue pool QE (0.150 +/- 16% [CV] micrograms/kg BW) is approximately 29% of Qtot. Furthermore, the size of liver pool QB and the size of kidney pool QC are about 10% and 2%, respectively, of Qtot. The rate of T3 metabolized in the skeletal muscle pool and in the unquantified composite tissue pool, as a sum, represents approximately 53% of the plasma appearance rate of the hormone (PAR3), while the rate of T3 metabolized in the liver and kidney pools represents 27% and 3% of PAR3, respectively. The rate of T3 excreted via feces and urine, as a sum, accounts for about 20% of PAR3.(ABSTRACT TRUNCATED AT 250 WORDS)