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A. B. lebodziski, E. Brzeziska-lebodziska

Utilizing the difference in pH optima between two distinct enzyme systems responsible for the thyroxine (T₄) inner and outer ring monodeiodination, and a specific radioimmunoassay for iodothyronines, the net T₄ conversion to 3,3',5'-tri-iodothyronine (rT₃) and 3,5,3'-tri-iodothyronine (T₃) in growing pigs kept under various food restrictions was determined in vitro. It has been found that fasting lowers deiodination of T₄ to T₃ by liver and kidney homogenates. The conversion of T₄ to rT₃ was reduced in the liver but not in the kidney, unless the animals were completely fasted. During food restriction serum T₄ and T₃ decreased whereas rT₃ increased, which indicated that the serum rT₃ level is not a good reflection of T₄ to rT₃ converting activity in the liver or kidney. The data suggest that a rise in rT₃ reflects a lowered degradation rate of the hormone or an increased secretion by the thyroid gland rather than changes in rT₃ neogenesis in the periphery.