Regulation of T3 production in the brain.

Intracerebral thyroid hormone metabolism is responsible for maintaining intracellular T3 levels within narrow limits. A coordinated regulation of brain T4 to T3 conversion and T3 degradation allows the brain to respond to changes in the availability of circulating T4 and thereby isolates the brain from environmentally influenced changes in thyroid hormone status. T4 to T3 conversion is regulated by dynamic increases or decreases in the cellular content of the enzyme that catalyzes this reaction that are due to changes in enzyme degradation not enzyme synthesis. Little is known about the molecular events regulating the other deiodinating enzymes in the brain. In addition to the dynamic regulation of T4 to T3 conversion, anatomical segregation of the deiodinases that activate and inactivate thyroid hormone contributes to the minimal contribution of circulating T3 to the intracerebral pool of bioactive thyroid hormone.