What does the Free Thyroxine (Free T4) test measure?

Why is Free Thyroxine (Free T4) measured?

Free T4 is the definitive second-tier thyroid marker, measured alongside TSH to confirm the nature, mechanism, and severity of thyroid dysfunction identified on TSH. Elevated Free T4 with suppressed TSH confirms hyperthyroidism; reduced Free T4 with elevated TSH confirms overt primary hypothyroidism; normal Free T4 with elevated TSH defines subclinical hypothyroidism; low or normal Free T4 with normal or low TSH suggests central (secondary) hypothyroidism from pituitary or hypothalamic pathology. Free T4 is particularly informative in patients on oestrogen therapy where TBG elevation renders Total T4 unreliable, and in the post-partum period where thyroid function fluctuates rapidly. Within the LabCorp Thyroid Panel, Free T4 provides the definitive confirmation of the degree of thyroid hormone production — the clinical decision marker for treatment initiation and dose titration.

Reference ranges

Standard reference

0.82–1.77 ng/dL (10.6–22.7 pmol/L) # REVIEWER FLAG: Confirm LabCorp reference range for Free T4 on this specific panel — confirm whether LabCorp uses a direct Free T4 immunoassay or a T3 Uptake/Free T4 Index (FTI) indirect calculation for this particular panel code. The two approaches produce different numerical results and require different interpretive frameworks. Direct immunoassay Free T4 values are clinically preferred and are unaffected by binding protein changes.

Functional medicine optimal

1.1–1.5 ng/dL (functional medicine mid-to-upper reference range optimal — levels in the lower half of the reference range may be associated with suboptimal peripheral thyroid hormone availability in symptomatic patients despite technically normal values). # REVIEWER FLAG: Not standardised in mainstream endocrinology — reviewer to confirm framing.

Units

ng/dL, pmol/L

Reference ranges vary by laboratory and assay platform. Interpret in context of the issuing laboratory's stated range.