Riboflavin metabolism in the hypothyroid human adult.

It had been shown that thyroxine regulates the conversion of riboflavin to riboflavin mononucleotide and flavin adenine dinucleotide (FAD) in laboratory animals. In the hypothyroid rat, the flavin adenine dinucleotide level of the liver decreases to levels observed in riboflavin deficiency. We have shown that in six hypothyroid human adults, the activity of erythrocyte glutathione reductase, an accessible FAD-containing enzyme, is decreased to levels observed during riboflavin deficiency. Thyroxine therapy resulted in normal levels of this enzyme while the subjects were on a controlled dietary regimen. This demonstrates that thyroid hormone regulates the enzymatic conversion of riboflavin to its active coenzyme forms in the human adult.