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Free T4 Thyroid Test: What Your Results Mean

Free T4 (thyroxine) is the main hormone produced by the thyroid gland. Unlike total T4, free T4 measures only the active, unbound portion available to the body's cells. It is used alongside TSH to diagnose and monitor thyroid disorders.

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Normal Range

0.8 – 1.8 ng/dL

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Unit

ng/dL

What Your Results Mean

Normal

Normal free T4 confirms that your thyroid is producing the right amount of hormone to support metabolism, energy, heart rate, and body temperature regulation.

High

High free T4 with low TSH indicates hyperthyroidism — the thyroid is overproducing hormone. Causes include Graves' disease, toxic nodules, or thyroiditis.

Low

Low free T4 with high TSH indicates hypothyroidism — the thyroid is not producing enough hormone. Usually caused by Hashimoto's thyroiditis or thyroid damage.

Frequently Asked Questions

What is a normal free T4 level? expand_more

Normal free T4 is 0.8–1.8 ng/dL, though ranges vary by laboratory. T4 must always be interpreted alongside TSH for accurate thyroid assessment.

What is the difference between TSH and T4? expand_more

TSH is produced by the pituitary gland and tells the thyroid to make T4. T4 is the actual thyroid hormone. Together they give a complete picture of thyroid function.

Can I have normal TSH but abnormal T4? expand_more

Yes — in early or mild thyroid dysfunction, TSH can still be within range while T4 is starting to shift. This is why both tests are often ordered together.

What conditions cause low free T4? expand_more

Hypothyroidism (Hashimoto's), thyroid surgery or radioiodine treatment, pituitary disorders, and severe illness can all cause low free T4.

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