Synonyms
Total Thyroxine
Clinical Significance
Total T4 testing measures the total amount of T4, including free T4 and T4 that is bound to proteins.
This test may be ordered to:
Follow up on an abnormal TSH test result
Diagnose hyperthyroidism, a condition involving an overactive thyroid gland
Diagnose hypothyroidism, a condition in which the thyroid gland is underactive
Monitor T4 levels in patients on thyroid replacement therapy
Screen for an underactive thyroid gland in newborn babies
Evaluate other disorders, such as goiters, thyroid nodules, and problems with the pituitary or hypothalamus gland
Specimen
Serum
Stability
7 days at 4 to 8 °C
30 days at -20 °C
72 hours at ambient temperature
Reference Range
57.9 – 150.6 nmol/L
Interpretation
| THYROID-STIMULATING HORMONE (TSH) | FREE T4 | THYROID FUNCTION |
|---|---|---|
| Normal | Normal | Normal Thyroid Function |
| High | Normal | Subclinical Hypothyroidism |
| High | High | Hyperthyroidism related to Pituitary gland |
| High | Low | Hypothyroidism related to the Thyroid gland |
| Low | Normal | Subclinical Hyperthyroidism |
| Low | High | Hyperthyroidism related to the Thyroid gland |
| Low | Low | Hypothyroidism related to Pituitary gland |
Limitation
The total T4 test is less reliable than free T4 because a number of factors may change the levels of protein in the body binding to T4. These protein levels—and therefore your total T4 level—can be affected by pregnancy, oral contraceptive use, severe illness, or the use of medicines called corticosteroids. Many healthcare providers prefer to use the free T4 test because it is not affected by changes in the levels of binding proteins and is therefore considered more accurate.