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Hormonal
Free T4
Thyroxine — the main thyroid hormone produced by the thyroid gland, converted to active T3 in tissues.
Optimal Range
1.0-1.5 ng/dL (optimal)
Why It Matters
Free T4 provides the substrate for active T3 production. Low levels indicate thyroid underproduction, while elevated levels may suggest hyperthyroidism or excess supplementation.