Biomarker guide
Free T4 (ng/dL)
Free T4 measures the unbound thyroxine circulating in your blood, the thyroid’s main output and the raw material for active T3 hormone. People test it with TSH to confirm whether the thyroid is actually delivering hormone, since TSH alone only shows the demand signal.
Reference ranges
Commonly used adult reference ranges. Labs vary in methodology and cutoffs, interpret your results against your lab report's stated ranges and with your doctor.
Commonly used reference ranges run roughly 0.8–1.8 ng/dL; Free T4 shows thyroid hormone supply, while TSH shows the pituitary’s demand signal.
| Zone | Range |
|---|---|
| Low | 0–0.8 ng/dL |
| Normal range | 0.8–1.8 ng/dL |
| High | 1.8–3 ng/dL |
The importance of Free T4
T4 is the hormone the thyroid gland principally makes, and the “free” fraction, the roughly 0.03% not bound to carrier proteins, is what tissues can take up and convert to active T3. Free T4 therefore answers the supply-side question that TSH cannot: is the gland producing?
Read together, the two markers form a simple grid. High TSH with low Free T4 fits an underactive thyroid; low TSH with high Free T4 fits an overactive one; an abnormal TSH with a still-normal Free T4 suggests an early or subclinical shift that is often just monitored. Estrogen therapy, pregnancy, biotin supplements, and some medications can distort results, which labs account for when flagged.
Free T4 is also the primary marker used to titrate thyroid hormone replacement alongside TSH. If your results pair an abnormal TSH with a Free T4 outside roughly 0.8–1.8 ng/dL, that combination, and any symptoms accompanying it, is worth discussing with your doctor.
Frequently asked questions
What is a normal Free T4 level?
Commonly used reference ranges for Free T4 run from about 0.8 to 1.8 ng/dL in adults, with modest variation between labs. Results are interpreted together with TSH, since the pairing distinguishes an underactive thyroid from an overactive one far better than either number alone.
Why test Free T4 instead of total T4?
Because over 99% of T4 is bound to carrier proteins and inactive, and carrier-protein levels swing with estrogen, pregnancy, liver disease, and some medications. Free T4 measures only the unbound, usable fraction, so it stays accurate when binding proteins shift.
What does low Free T4 with high TSH mean?
That pattern is the classic signature of an underactive thyroid: the pituitary is calling for more hormone (high TSH) and the gland is not delivering (low Free T4). It is one of the clearer patterns in thyroid testing and warrants a conversation with your doctor about causes and next steps.
Can supplements interfere with Free T4 results?
Biotin is the classic offender, high-dose biotin (common in hair and nail supplements) can interfere with many thyroid immunoassays, producing falsely high Free T4 and falsely low TSH readings. Most labs suggest pausing biotin for at least 48 hours before a thyroid blood draw.
Panels that measure Free T4
Thyroid Function Blood Test Panels: TSH, Free T4 & Free T3
From a basic TSH and T4 screen to a full antibody workup, thyroid panels for every level of detail.
View panelThyroid Blood Test Panels: TSH, Free T4, Free T3 & Antibodies
From a basic TSH check to a full antibody workup, a complete menu of thyroid tests and panels, currently 20% off.
View panel