Free T4 (Free Thyroxine)
Free T4 measures only the thyroxine that is not bound to proteins in the blood — the fraction that is biologically active and available for tissues to use. It is more accurate than total T4 because it is not affected by changes in binding protein levels (which occur in pregnancy, liver disease, and with certain medications).
Reference Ranges
Male
0.8 – 1.8
ng/dL
Female
0.8 – 1.8
ng/dL
Child
0.8 – 2
ng/dL
Ages 1–12
Reference ranges may vary by laboratory. Always compare with the range printed on your lab report.
What Low Levels Mean
Low free T4 with high TSH is the classic pattern of primary hypothyroidism. Low free T4 with low or normal TSH suggests secondary hypothyroidism (pituitary gland problem) — a rarer but important condition that requires brain imaging.
What High Levels Mean
High free T4 with low TSH indicates hyperthyroidism. High free T4 with normal or high TSH is unusual and may indicate TSH-secreting pituitary tumor or thyroid hormone resistance — both rare conditions requiring specialist evaluation.
Frequently Asked Questions
Is free T4 the same as T4?
No. Total T4 includes both bound (99.97%) and free (0.03%) fractions. Only the free fraction is active. Total T4 can be misleadingly high or low due to changes in binding proteins, while free T4 reflects true thyroid status.
What is the ideal free T4 level on thyroid medication?
Most patients feel best when free T4 is in the upper half of the normal range (1.2–1.8 ng/dL) with TSH in the lower normal range (0.5–2.5 mIU/L). However, the optimal level varies per person — symptom relief is more important than a specific number.
Can free T4 be normal but TSH still high?
Yes — this is called subclinical hypothyroidism. The thyroid is struggling (hence high TSH) but still managing to produce enough hormone (normal free T4). Treatment depends on how high TSH is, symptoms, and whether you are trying to conceive.
This information is for educational purposes only and should not replace professional medical advice. Always consult your doctor for interpretation of your test results.
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