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)
Risk-Stratified Targets
| Population / Context | Target |
|---|---|
| Optimal | 1.0–1.5 ng/dL |
| Normal lab range | 0.8–1.8 ng/dL |
| Low (hypothyroid) | < 0.8 ng/dL |
| Elevated (hyperthyroid or over-replacement) | > 1.8 ng/dL |
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.
Understanding Free T4
Free T4 (thyroxine) is the primary hormone produced by the thyroid gland, accounting for approximately 80% of thyroid hormone output. It circulates in the blood largely bound to proteins, with only the 'free' (unbound) fraction being biologically available. Free T4 serves primarily as a reservoir and precursor — it must be converted to T3 in peripheral tissues to activate metabolic processes.
In clinical practice, Free T4 is most useful when interpreted alongside TSH. A low Free T4 with elevated TSH confirms primary hypothyroidism (the thyroid is underproducing). A low Free T4 with low or normal TSH may indicate central hypothyroidism (a pituitary problem) — a less common but important diagnosis that standard TSH-only screening can miss.
For patients on thyroid hormone replacement (levothyroxine/T4), Free T4 levels help assess dosing adequacy. The mid-to-upper portion of the reference range is generally the target, though individual optimization should also consider Free T3, TSH, and symptom resolution. In longevity medicine, maintaining robust T4 levels ensures adequate substrate for T3 conversion, supporting metabolic rate, cognitive function, and overall vitality.