The enzyme HMG-CoA reductase exists in two interconvertible forms. The dephosphorylated form of HMG-CoA reductase is more active while the phosphorylated is less active. The hormones exert their influence through cAMP by a series of reactions, which are comparable with the control of the enzyme glycogen synthase. The net effect is that glucagon and glucocorticoids favour the formation of inactive HMG-CoA reductase (phosphorylated form) and thus decrease cholesterol synthesis. On the other hand, insulin and thyroxine increase cholesterol production by enhancing the formation of HMG-CoA reductase (dephosphorylated form).