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Free Thyroxine Optimization

Meaning

Free thyroxine optimization is the clinical strategy focused on achieving and maintaining the ideal circulating concentration of unbound thyroxine (fT4), the primary thyroid hormone precursor. This optimization aims for a level within the upper-middle range of the physiological reference interval, ensuring adequate substrate for conversion to the biologically active triiodothyronine (fT3) at the cellular level. Because only the unbound fraction is metabolically active, precise management of fT4 is crucial for maintaining systemic metabolic rate, thermogenesis, and neurocognitive function. Suboptimal fT4 levels, even within the laboratory’s broad ‘normal’ range, can lead to subtle hypometabolic states.