Clinical Studies on Metabolism of Thyroid Hormone

The metabolism of triiodothyronine was studied using I131-labeled 1-triiodothyronine on normal subjects and on patients with various thyroid disorders and the metabolism of triiodothyronine was compared with that of thyroxine.1) For 3-5 days after intravenous administration of radiotriiodothyronine 50μc (3/μg of 1-triiodothyronine), serum I131 concentration, urinary I131 excretion, fecal I131 excretion and the radioactivity over the thyroid gland, liver and thigh, were measured daily. All subjects were given natorium iodide by mouth daily for two or more days prior to and throughout the course of the studies in order to block the uptake by the thyroid gland of radioiodide liberated during the peripheral degradation of radiotriiodothyronine. As the radioactivity over the thyroid gland, liver and thigh diminished at almost the same exponential rate as the serum I131 concentration, it was considered that, during the peripheral degradation of radiotriiodothyronine, radioiodine accumulated in two sites : the urine and feces. Hence, Daily Volume Turnover (DVE) (1/day) =U+F. U, urinary portion (1/day) and F, fecal portion (1/day) were calculated according to the same formulas as calculation of thyroxine degradation rate in Part I.2) When the DVT was plotted against the PBI value for normal subjects and for patients with various thyroid disorders. The DVT was found to be roughly proportional to the PBI value. By the method of least squares the relation between PBI (μg/dl) and DVT (1/day) was expressed by the following formula : DVT = 3.3608 + 1.0309 × (PBI) 3) From the comparison of this experimental formula and that in metabolic studies with radiothyroxine it was concluded that metabolic speed of triiodothyronine averaged about 6 times faster than that of thyroxine.