Free T4 (fT4)
Stomach and gut healthFree T4 is produced by the thyroid gland and converted to active T3 in body tissues.
Why this matters
Free T4 reflects the thyroid gland's production capacity and serves as the primary screening test for thyroid dysfunction. It provides early detection of both hypothyroidism (low fT4) and hyperthyroidism (high fT4) before symptoms become obvious. Because it's unaffected by protein-binding changes, free T4 gives a true picture of thyroid hormone availability, helping distinguish between thyroid disease and conditions that merely affect hormone transport.
How this connects to other biomarkers
- Low fT4 with elevated TSH is the cardinal pattern of underactive thyroid (primary hypothyroidism); with low or normal TSH suggests a brain/pituitary cause (central hypothyroidism).
- High fT4 with suppressed TSH confirms overactive thyroid (primary hyperthyroidism); with normal/elevated TSH points to a TSH-producing pituitary tumor or thyroid hormone resistance.
- The fT4 / TSH ratio can highlight subtle thyroid axis disturbances when each marker is borderline.
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