Free T4
Free T4 (thyroxine) is the unbound, biologically available form of the primary thyroid hormone. The thyroid produces mostly T4, which serves as a prohormone — it is converted to the more active T3 in peripheral tissues (liver, kidneys, muscle). Only 0.03% of total T4 is free; the rest is bound to proteins (TBG, albumin).
Why it matters: Free T4 is a core measure of thyroid hormone production. Low free T4 with high TSH is the pattern commonly seen in hypothyroidism; high free T4 with low TSH is the pattern seen in hyperthyroidism. Free T4 is more stable than free T3 and less affected by acute illness, so it is often measured alongside TSH.
May increase with:
May decrease with:
What is Free T4?
Free T4 (thyroxine) is the unbound, biologically available form of the primary thyroid hormone. The thyroid produces mostly T4, which serves as a prohormone — it is converted to the more active T3 in peripheral tissues (liver, kidneys, muscle). Only 0.03% of total T4 is free; the rest is bound to proteins (TBG, albumin).
What might a high or low Free T4 mean?
Free T4 is a core measure of thyroid hormone production. Low free T4 with high TSH is the pattern commonly seen in hypothyroidism; high free T4 with low TSH is the pattern seen in hyperthyroidism. Free T4 is more stable than free T3 and less affected by acute illness, so it is often measured alongside TSH.
What is the typical reference range for Free T4?
The general-population reference range shown here is 12 – 22 pmol/L. Reference ranges describe the general population and are not a personal target — discuss your results with your physician.
What can affect Free T4?
It may be higher with: Graves' disease, Toxic thyroid nodule, Thyroiditis (destructive release), Excessive thyroid hormone replacement, Amiodarone (iodine-induced), Biotin supplements (assay interference — falsely elevated). It may be lower with: Hashimoto's thyroiditis (autoimmune destruction), Iodine deficiency, Post-surgical or post-radioiodine, Central hypothyroidism (pituitary failure), Non-thyroidal illness (sick euthyroid syndrome), Medications (lithium, amiodarone).