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Free thyroxine (FT4) comprises a small fraction of total thyroxine. FT4 is available to the tissues and is, therefore, the metabolically active fraction.Elevations in FT4 cause hyperthyroidism, while decreases cause hypothyroidism. Pediatric 0-5 days: 0.9-2.5 ng/dL, 6 days-2 months: 0.9-2.2 ng/dL, 3-11 months: 0.9-2.0 ng/dL, 1-5 years: 1.0-1.8 ng/dL, 6-10 years: 1.0-1.7 ng/dL, 11-19 years: 1.0-1.6 ng/dL Adult (> or =20 years): 0.9-1.7 ng/dL
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