Free T4: What Your Thyroid Hormone Result Means
Free T4 is the unbound thyroid hormone available to your tissues. Learn how it pairs with TSH, what high or low means, and what can skew it.
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What it is
T4 (thyroxine) is the main hormone your thyroid gland releases. Most of it circulates bound to proteins; free T4 measures the small unbound fraction that tissues can actually use.
Free T4 is interpreted with TSH, the pituitary signal that tells the thyroid to work harder or ease off. The pattern of the two together is what defines a thyroid problem.
Why it matters
High TSH with low free T4 indicates overt hypothyroidism; high TSH with normal free T4 is subclinical hypothyroidism, which is often watched rather than treated immediately.
Low TSH with high free T4 indicates hyperthyroidism, which can cause palpitations, weight loss, heat intolerance, and anxiety.
Typical reference ranges
Ranges vary by lab and individual circumstances. These are common adult reference ranges for orientation only.
| Group | Common range |
|---|---|
| Normal | 0.8–1.8 ng/dLLab ranges vary; use your report’s range |
| Low | < 0.8 ng/dLWith high TSH, suggests hypothyroidism |
| High | > 1.8 ng/dLWith low TSH, suggests hyperthyroidism |
| TSH reference | 0.4–4.0 mIU/L |
Not sure where your free t4 falls? Upload your lab report and Nodava marks it against the range for you.
See where you fallWhat can move your result
Biotin supplements can badly distort thyroid assays and should be stopped several days before testing. Acute illness, pregnancy, estrogen, amiodarone, lithium, and steroids also affect results.
Frequently asked questions
Can free T4 be normal with thyroid symptoms?
Does biotin really affect the test?
Should free T3 be tested too?
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Free T4 is just one piece of the puzzle. Nodava explains every biomarker on your report, highlights what is out of range, and helps you track changes over time.
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