Lab test

Free T3 Levels Test - Thyroid Testing (FT3)

This blood test measures free triiodothyronine (Free T3), the unbound and biologically active form of T3.

Typical price

From $198

Results

1-3 business days

Preparation

See preparation notes below

Sample

Blood draw or sample collection at a local center

This blood test measures free triiodothyronine (Free T3), the unbound and biologically active form of T3. T3 is the thyroid hormone that acts most directly on cells to regulate metabolism, energy use, heart rate, and body temperature. Most T3 is converted from T4 in peripheral tissues, and only the free portion can enter cells. Free T3 is less affected by changes in binding proteins than total T3, so it can give a more reliable picture of thyroid activity. The test is ordered to investigate suspected an overactive thyroid, to assess T3 deficiency, and to monitor certain thyroid conditions. It is interpreted with TSH and Free T4.

• Free T3 (Free Triiodothyronine) The unbound, active form of triiodothyronine available to cells.

• People with symptoms of an overactive thyroid
• Individuals being evaluated for T3 deficiency
• Those with abnormal TSH or Free T4 results
• People being monitored during thyroid treatment

• Report thyroid medications, biotin, and other supplements to the laboratory
• Follow any timing instructions provided with your order
• Tell your provider if you are pregnant or recently ill

Free T3 is compared against reference ranges that may depend on age and pregnancy status. A high level can accompany an overactive thyroid, while a low level may occur with illness, certain conditions, or reduced conversion of T4 to T3. Because Free T3 can be affected by non-thyroid illness, it is interpreted with TSH and Free T4 rather than alone. A healthcare provider should assess the result with your symptoms and any further testing.

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