Free T4 measures the unbound, active form of thyroxine - the main hormone produced by your thyroid gland. Unlike total T4, free T4 is not affected by protein levels and gives a more accurate picture of thyroid function.
Free T4 (free thyroxine) is the portion of thyroid hormone T4 that circulates unbound to proteins in your blood. Only about 0.03% of T4 is "free" - the rest is bound to carrier proteins. This free fraction is what's actually available for your cells to use.
Because free T4 isn't affected by protein binding changes (which can occur with pregnancy, estrogen therapy, liver disease, or certain medications), it provides a more accurate measure of your true thyroid hormone status than total T4.
Free T4 is typically tested alongside TSH to get a complete picture of thyroid function. While TSH tells you how hard your pituitary is working to stimulate the thyroid, free T4 shows you how much active hormone your thyroid is actually producing.
Price
$9
Results
1-2 days
Lab Network
Quest
Fasting
Not Required
What's Measured
What This Test Measures
This test measures the concentration of unbound, biologically active T4 in your blood, reported in ng/dL. Normal free T4 levels typically range from 0.8-1.8 ng/dL, though this can vary slightly by laboratory. Low levels suggest hypothyroidism while high levels suggest hyperthyroidism.
Who Should Consider This Test
Anyone with abnormal TSH who needs further evaluation, people on thyroid medication (to monitor therapy), those with symptoms of thyroid dysfunction, pregnant women (thyroid needs increase during pregnancy), and anyone taking medications that affect thyroid binding proteins.
Health Insights
Conditions Screened
Related Symptoms
When to Get Tested
No fasting required. Can be tested any time of day. If taking thyroid medication, test before your daily dose or at a consistent time for monitoring. Often ordered with TSH for complete assessment.
Understanding Your Results
Normal free T4 is typically 0.8-1.8 ng/dL. Low free T4 with high TSH indicates primary hypothyroidism. High free T4 with low TSH indicates hyperthyroidism. Low free T4 with low/normal TSH may suggest pituitary problems. Results should be interpreted with TSH for complete picture.
What to Expect
After placing your order, you'll receive an email from Rupa Health (our lab order partner) within 24 business hours to sign a consent form and schedule your blood draw at a Quest Diagnostics location.
The blood draw typically takes just a few minutes. Results are usually available within 1-2 business days and will appear in your secure Justlabs dashboard.
Our results include reference ranges and easy-to-understand indicators showing whether your values are within normal limits.
Frequently Asked Questions
$9
Typical hospital cash price: $120–$180
+ $28 in standard lab & physician fees at checkout (no additional bills later)
$18 lab collection + $10 physician order
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