Free T3 measures the unbound, most active form of thyroid hormone. T3 is 3-4 times more potent than T4 and directly affects metabolism, heart rate, body temperature, and energy levels.
Triiodothyronine (T3) is the most active thyroid hormone, being 3-4 times more potent than T4. While the thyroid produces some T3 directly, most T3 is converted from T4 in peripheral tissues like the liver, kidneys, and muscles.
Free T3 measures the unbound, biologically active portion of T3 in your blood. This free fraction (about 0.3% of total T3) is what actually enters cells and regulates metabolism, body temperature, heart rate, and energy production.
Testing free T3 is particularly important when:
- Hyperthyroidism is suspected (T3 often rises before T4)
- Symptoms persist despite normal TSH and T4
- T4-to-T3 conversion problems are suspected
- Evaluating thyroid medication that contains T3
Price
$12
Results
1-2 days
Lab Network
Quest
Fasting
Not Required
What's Measured
What This Test Measures
This test measures the concentration of unbound, active T3 in your blood, reported in pg/dL. Normal free T3 levels typically range from 2.3-4.2 pg/dL. Low levels can indicate hypothyroidism or poor T4-to-T3 conversion, while high levels suggest hyperthyroidism.
Who Should Consider This Test
People with hyperthyroid symptoms (especially rapid heart rate, weight loss, anxiety), those with persistent fatigue despite normal T4, patients on T3-containing thyroid medications, and anyone with suspected T4-to-T3 conversion issues.
Health Insights
Conditions Screened
Related Symptoms
When to Get Tested
No fasting required. Can be tested any time of day. If taking T3-containing medication (Cytomel, Armour, compounded), timing relative to dose matters - discuss with your provider.
Understanding Your Results
Normal free T3 is typically 2.3-4.2 pg/dL. High free T3 with low TSH indicates hyperthyroidism. Low free T3 with high TSH suggests hypothyroidism. Low free T3 with normal T4 may indicate conversion problems from chronic illness, stress, or certain medications.
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
$12
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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