The peripheral metabolism of triiodothyronine in normal subjects and in patients with hyperthyroidism

In order to assess the contribution of 3.3',5-triiodo-L-thyronine (T(3)) to overall thyroid hormone economy, conjoint measurements of the kinetics of peripheral T(3) metabolism and the total concentration of T(3) in serum were made in a group of normal subjects and in a group of patients with hyperthyroid Graves' disease. As judged from the disappearance of trichloroacetic acid-precipitable (131)I from serum after a single intravenous dose of labeled T(3), the following mean values were obtained in the normal subjects: volume of distribution, 43 liters or 0.62 liter/kg; fractional turnover rate. 52% per 24 hr: clearance rate, 22.3 liters/24 hr: and absolute disposal rate, 60 mug/24 hr. In the patients with untreated hyperthyroidism, values for all these functions were greatly increased. After treatment, the volume of T(3) distribution returned to normal but the fractional turnover rate remained abnormally rapid.