Free T4 Test Results Explained: What Your Number Means

Free T4 results, explained

Enter your value to see whether it falls in the typical range and what that means. This is an educational explainer — not a diagnosis, and your entry never leaves your browser.

Enter your Free T4 value

ng/dL

Some labs report in pmol/L; the reference range will differ — use your report's range.

This is educational, not a diagnosis. Reference ranges vary between labs and assays — the range printed on your report is the one that applies to you, and a single value is interpreted with your history and symptoms by a clinician. Your entry stays in your browser; nothing is stored or sent anywhere.

Reference ranges: American Thyroid Association — Thyroid Function Tests. View source

What Free T4 measures

Free T4 is the unbound, active form of the main thyroid hormone thyroxine circulating in your blood.

Why the test is done

Usually alongside TSH to clarify thyroid function when TSH is abnormal.

Typical reference ranges

Typical adult reference bands for Free T4 (ng/dL)
Band	What it may mean
Below typical range	A low free T4 with a high TSH is the classic pattern of an underactive thyroid (hypothyroidism). It's interpreted together with TSH by a clinician.
Within typical range	This is the commonly used adult range for free T4. Paired with a normal TSH, thyroid function is usually considered normal.
Above typical range	A high free T4 with a low TSH suggests an overactive thyroid (hyperthyroidism). Your clinician interprets it with TSH and symptoms.

Ranges shown are typical adult values from American Thyroid Association; your own lab's printed range applies to you. View source.

Frequently asked questions

What is a normal free T4 level?

A typical adult range is roughly 0.8 to 1.8 ng/dL, but ranges vary by lab and assay — always compare against the range printed on your own report.