Hepatic conversion of amino nitrogen to urea nitrogen in hypothyroid patients and upon L-thyroxine therapy.
Conflicting studies have been reported regarding the influence of thyroid hormones on hepatic nitrogen metabolism and liver metabolic activity. We studied urea N synthesis rate (UNSR), functional hepatic N clearance (FHNC), galactose elimination capacity, and antipyrine clearance in six hypothyroid female patients before and after achievement of a stable euthyroid status. In both conditions, UNSR measured at intervals in response to constant alanine infusion was linearly related to the average alpha-amino N concentrations. In the hypothyroid state, peak UNSR was decreased by 31% in comparison with values measured in euthyroidism, which were in the normal range. FHNC (ie, the slope of the linear relation between UNSR and blood alpha-amino N concentration) is a measure of the kinetics of the process of hepatic amino N to urea N conversion; it was 19.8 +/- 4.0 L.h-1 in hypothyroid patients and increased to normal values after L-thyroxine replacement (30.4 +/- 3.3 L.h-1, P < .01; normal values > 25 L.h-1). Hepatic microsomal and cytosolic activities (antipyrine clearance and galactose elimination) were normal in hypothyroid patients and did not change significantly after therapy. Our data show a specific defect in hepatic handling of amino acids in hypothyroid patients, leading to reduced alpha-amino N to urea N conversion, in the absence of any detectable impairment in different hepatic metabolic activities.
Marchesini
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Istituto di Clinica Medica Generale e Terapia Medica, Università di Bologna, Italy.
Fabbri
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Bianchi
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Motta
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Bugianesi
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Urbini
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Pascoli
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Lodi
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D000328:Adult; D000409:Alanine / Q000494:pharmacology; D000596:Amino Acids / Q000378:metabolism*; D005260:Female; D005690:Galactose / Q000378:metabolism; D006706:Homeostasis; D006801:Humans; D007037:Hypothyroidism / Q000188:drug therapy / Q000378:metabolism*; D016014:Linear Models; D008099:Liver / Q000378:metabolism*; D008875:Middle Aged; D009584:Nitrogen / Q000378:metabolism*; D013974:Thyroxine / Q000627:therapeutic use*; D014508:Urea / Q000378:metabolism*
D000596:Amino Acids; D014508:Urea; D009584:Nitrogen; D000409:Alanine; D013974:Thyroxine; D005690:Galactose