Free T4 Test
Also known as: FT4, Free Thyroxine
What is Free T4?
Free T4 is the unbound, active form of thyroxine. It is the main hormone produced by the thyroid gland and is converted to T3 in tissues.[MedlinePlus (NIH)]
What This Test Measures
This test measures the free (unbound) T4 in your blood. Free T4 is the portion available to enter cells and exert effects.[Mayo Clinic]
Why It's Important
Free T4 is often the most useful thyroid hormone test because it is not affected by protein levels.[Cleveland Clinic]
Abnormal Free T4 with abnormal TSH confirms thyroid dysfunction.[MedlinePlus (NIH)]
Who Should Get This Test?
Free T4 is typically ordered alongside TSH to evaluate thyroid function.
Risk Factors
- Abnormal TSH result requiring confirmation
- Symptoms of hyperthyroidism or hypothyroidism
- Monitoring thyroid hormone replacement therapy
- Suspected pituitary or hypothalamic disease
- Pregnancy (to monitor thyroid function)
- Taking medications affecting thyroid binding proteins
- Patients with conditions that alter protein levels (liver disease, nephrotic syndrome)
Screening Schedule
Not used as a primary screening test. Ordered when TSH is abnormal or to monitor treatment.
What Happens During the Test
A blood sample is drawn from a vein in your arm. The lab measures unbound thyroxine levels.
Duration
5 minutes for the blood draw; results usually available within 1-2 days
Discomfort Level
Minimal - brief pinch from needle insertion
Risks & Side Effects
Free T4 testing through blood draw carries very few risks.
- •Minor pain or bruising at the needle site
- •Slight bleeding
- •Rarely, lightheadedness
- •Very rare risk of infection
Normal Range
0.8 to 1.8 ng/dL.
Note: Ranges vary by laboratory and during pregnancy.
Source: Cleveland ClinicUnit Conversion
Convert between ng/dL and pmol/L
| ng/dL | pmol/L |
|---|---|
| 0.6 | 7.7 |
| 0.8 | 10.3 |
| 1 | 12.9 |
| 1.4 | 18 |
| 1.8 | 23.2 |
| 2.2 | 28.3 |
High Values
High Free T4 indicates hyperthyroidism. Causes include Graves disease, toxic nodular goiter, and thyroiditis.[Mayo Clinic]
Low Values
Low Free T4 indicates hypothyroidism. Causes include Hashimoto thyroiditis, thyroid surgery, and iodine deficiency.[Mayo Clinic]
Test Limitations & Accuracy
Free T4 is generally reliable but has some limitations.
- ⚠Some assay methods can be affected by severe illness
- ⚠Biotin supplements can interfere with certain test methods
- ⚠Pregnancy requires trimester-specific reference ranges
- ⚠Should be interpreted together with TSH, not in isolation
- ⚠Some medications (heparin, phenytoin) may affect results
Alternative & Complementary Tests
Free T4 is typically part of comprehensive thyroid evaluation.
First-line test for thyroid screening
When Free T4 assay may be unreliable
When hyperthyroidism is suspected but Free T4 is normal
To diagnose autoimmune thyroid disease
References
Related Condition Guides
Medical Disclaimer
This information is for educational purposes only and is not intended as medical advice. Always consult with a qualified healthcare provider for interpretation of test results and medical decisions.
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