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The Free T4 (Free Thyroxine) test measures the level of unbound, biologically active T4 — the primary hormone produced by the t...
Also known as: FT4 Test · Free Thyroxine Test · Free T4 Level Test · Unbound T4 Test
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*Inclusive of home collection charges
The Free T4 (Free Thyroxine) test measures the level of unbound, biologically active T4 — the primary hormone produced by the thyroid gland — in the blood. Unlike total T4, Free T4 is unaffected by changes in carrier protein levels, making it the most accurate measure of thyroid hormone availability in complex clinical situations including pregnancy, liver disease, and nephrotic syndrome. Free T4 is the standard confirmatory test ordered alongside TSH when TSH is abnormal, and is the preferred T4 measure for thyroid monitoring in pregnancy. No fasting required.
Key benefits of this package include:
What Does the Free T4 (Free Thyroxine) Test Measure?
| Level / Result | Possible Interpretation |
|---|---|
| Normal FT4 (0.8–1.8 ng/dL) + Normal TSH | Euthyroid — normal thyroid function |
| Low FT4 + Elevated TSH | Overt primary hypothyroidism — levothyroxine replacement therapy required |
| Normal FT4 + Elevated TSH | Subclinical hypothyroidism — thyroid beginning to underperform; monitor or treat based on TSH level and anti-TPO status |
| Elevated FT4 + Low/suppressed TSH | Overt hyperthyroidism — Graves' disease, toxic nodule, or thyroiditis; antithyroid management required |
| Normal FT4 + Low/suppressed TSH | Subclinical hyperthyroidism or physiological (1st trimester of pregnancy) — clinical correlation required |
| Low FT4 + Low/normal TSH | Central hypothyroidism — pituitary or hypothalamic failure; specialist endocrinology evaluation required |
Important Note: Free T4 is most meaningful when interpreted alongside TSH. FT4 alone is insufficient for diagnosing subclinical thyroid disease — TSH is more sensitive and should always be included. In pregnancy, trimester-specific reference ranges must be applied. All results must be reviewed by a physician or endocrinologist.
Central Hypothyroidism — When TSH Is Misleadingly Normal or Low: In standard primary thyroid disease, low thyroid hormones cause TSH to rise (compensatory response). But in central hypothyroidism — caused by pituitary or hypothalamic failure — the pituitary cannot secrete TSH, so both FT4 and TSH are low simultaneously. Relying on TSH alone would miss central hypothyroidism entirely, as TSH appears normal or low rather than elevated. Measuring FT4 alongside TSH catches this important diagnosis — which requires different management (pituitary investigation) than primary hypothyroidism.
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