Possible arterio-venous gradients of T 4 , T 3 , rT 3 and 3,3'-diiodothyronine (3,3'-T 2 ) across the liver and the kidneys were measured in 9 patients with varying degrees of liver failure undergoing diagnostic catheterization. Plasma iodothyronine levels were measured in peripheral, hepatic and renal veins before and at 10-min intervals until 60 min after iv injection of 400 μg of TRH. In 2 patients estimated hepatic plasma flow and effective renal plasma flow were determined as well. In these 2 patients, no significant differences between iodothyronine levels in arterial and peripheral venous plasma were found. T 4 and T 3 levels were not significantly different between peripheral, renal and hepatic veins. Hepatic vein rT 3 and 3,3'-T 2 concentrations were 10.7 ± 8.3% (mean ± sd , P < 0.005) and 36 ± 18% ( P < 0.001) lower than those in the peripheral vein (N = 9). Renal vein rT 3 was just (6.2 ± 7.5%, P < 0.05) lower than rT 3 in peripheral vein, whereas 3,3'-T 2 was not different between the two veins. Estimates of hepatic and renal plasma flow were in agreement with values from the literature. On the basis of these data approximate hepatic clearance rates of 110 and 380 1/day for rT 3 and 3,3'-T 2 and a renal clearance rate of about 35 1/day for rT 3 were calculated. Sixty min after TRH, plasma T 3 was increased to 147 ± 56% ( P < 0.05) and 3,3'-T 2 in peripheral plasma was increased to 142 ± 36% ( P < 0.025), whereas plasma T 4 and rT 3 did not change. These data suggest that the liver is the major site of plasma rT 3 and 3,3'-T 2 clearance in patients with mild liver disease and probably also in healthy humans. The kidneys may account for roughly 25% of plasma rT 3 clearance. The increase in plasma 3,3'-T 2 after TRH may be secondary to the rise in precursor T 3 , but thyroidal secretion of 3,3'-T 2 is not excluded.
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Bauer et al. (1987) studied this question.