The Effect of Thyroid Hormones on Riboflavin Metabolism

The activities of a variety of flavin-dependent enzymes are depressed in hypothyroidism. They are increased by the administration of thyroxine or triiodothyronine, as a result of increased synthesis of riboflavin phosphate and 

leading to increased saturation of enzyme proteins with coenzymes. This increases the stability of the enzymes against proteolysis and increases their activity in tissues (Rivlin and Langdon, 1966). 

Tissue concentrations of flavin coenzymes in hypothyroid animals may be as low as in those fed a riboflavin-deficient diet, in hypothyroid patients, erythrocyte glutathione reductase (EGR) activity may be as low, and its activation by FAD added in vitro (Section 7.5.2) as high, as in riboflavin-deficient subjects. Tissue concentrations of flavin coenzymes and EGR are normalized by the administration of thyroid hormones, with no increase in riboflavin intake (Cimino et al., 1987). 

The administration of thyroid hormones to hypothyroid animals results in a rapid increase in flavokinase activity as a result of the activation of an inactive precursor protein as flavokinase activity increases, there is a parallel decrease in the tissue content of an apparendy inactive riboflavin binding protein (Lee and McCormick, 1985). 

Hyperthyroidism is not associated with elevated tissue concentrations of flavin coenzymes, despite increased activity of flavokinase. Again, this demonstrates the importance of the enzyme binding of flavin coenzymes and the rapid hydrolysis of unbound FAD and riboflavin phosphate in the regulation of tissue concentrations of the vitamin. 

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