Educational information only. At Precursor, blood testing is requested and reviewed by a clinician and considered alongside your history and other markers, not as a stand-alone diagnosis. Testing is performed in NATA-accredited laboratories to ISO 15189 standards.
What is free T4?
Free T4, or free thyroxine, is the main hormone the thyroid releases. Most T4 in the blood is bound to carrier proteins and inactive; the small 'free' fraction is the part available to act on your tissues. Measuring free T4 rather than the total gives a truer read of how much active hormone your body actually has to work with.
Why it matters
Free T4 shows how much hormone the thyroid is producing, and pairs naturally with TSH to locate where any imbalance sits. Together they let a clinician tell an underactive thyroid from an overactive one, and distinguish a thyroid problem from a pituitary one.
What your results can mean
If your result is higher
A high free T4 generally points toward an overactive thyroid, and is usually accompanied by a low TSH. It is interpreted alongside your symptoms and other thyroid markers.
If your result is lower
A low free T4 generally points toward an underactive thyroid, often with a raised TSH. Context and repeat testing help confirm the picture.
Australian reporting and interpretation
Free T4 is reported in picomoles per litre (pmol/L). The RCPA guide interval is 10 to 25 pmol/L and is method dependent. Your result should be read against the interval on your report and alongside TSH.
Test your thyroid with Precursor
Free T4 is most informative paired with TSH. At Precursor it is measured as part of Full Advanced Bloods, a single clinician-reviewed blood draw covering your thyroid, heart, metabolic, hormonal, kidney, liver and nutrient markers together, giving you and your clinician a complete thyroid picture and a baseline to track.
