The intracellular conversion of T 4 to T 3 was investigated in various tissues of hypothyroid rats after continuous iv infusion of radiolabelled T 3 and T 4 . Two groups of 4 thyroidectomized rats were infused with carrier-free 125 I-labelled T 4 as well as 131 I-labelled T 3 until isotope equilibrium was achieved. Plasma, various tissue homogenates (liver, kidney, pituitary, thigh muscle, cerebral cortex and cerebellum) and subcellular fractions (nuclei, mitochondria, microsomes, cytoplasm) from liver, kidney and the pituitary gland were extracted for thin layer chromatography. The [ 125 I]T 3 /[ 131 I]T 3 ratios were determined and the extra contribution of [ 125 I]T 3 derived from local conversion of [ 125 I]T 4 to the total [ 125 I]T 3 was calculated in percent. In addition to the [ 125 I]T 3 derived from plasma, [ 125 I]T 3 derived from locally converted [ 125 I]T 4 was present in all tissues investigated. There was substantially more, although in varying quantities, in the cerebral cortex (79 ± 2%), the cerebellum (68 ± 4%) and the pituitary gland (53 ± 1%) than in the liver (10 ± 6%), the kidney (11 ± 5%) and thigh muscle (17 ± 6%); in the latter tissues most of the 125 I-labelled T 3 is derived directly from plasma. These results indicate that in the brain of severe hypothyroid rats there is pronounced conversion of T 4 to T 3 and effective binding of the T 3 produced whereas the T 3 in the liver, kidney, and muscle is predominantly derived from plasma. At the intracellular level, within the investigated tissues, the locally formed T 3 was distributed equally over the subcellular fractions.
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Doom et al. (1982) studied this question.