Catalyzes the NAD(+)-dependent oxidation of L-threonine to 2-amino-3-ketobutyrate, mediating L-threonine catabolism. L-threonine + NAD(+) = (2S)-2-amino-3-oxobutanoate + H(+) + NADH Amino-acid degradation; L-threonine degradation via oxydo-reductase pathway; glycine from L-threonine: step 1/2. Homodimer. Belongs to the NAD(P)-dependent epimerase/dehydratase family.