When to order free T4 test ?
Free thyroxine (free T4) tests are used to help evaluate thyroid function and diagnose thyroid diseases, including hyperthyroidism and hypothyroidism, usually after discovering that the thyroid stimulating hormone (TSH) level is abnormal.
T4 and another hormone called triiodothyronine (T3) are produced by the thyroid gland. They help control the rate at which the body uses energy and are regulated by a feedback system. TSH stimulates the production and release of T4 (primarily) and T3 from the thyroid gland.
Most of the T4 and T3 circulates in the blood bound to protein, while a small percentage is free (not bound). Blood tests can measure total T4, free T4, total T3, or free T3. The total T4 test has been used for many years, but it can be affected by the amount of protein available in the blood to bind to the hormone. Free T4 is not affected by protein levels and is the active form of thyroxine. The free T4 test is thought by many to be a more accurate reflection of thyroid hormone function and, in most cases, its use has replaced that of the total T4 test.
What does the test result mean?
In general, high free T4 results may indicate an overactive thyroid gland (hyperthyroidism), and low free T4 results may indicate an underactive thyroid gland (hypothyroidism). The test results alone are not diagnostic but will prompt the health practitioner to perform additional testing to investigate the cause of the excess or deficiency.
Both decreased and increased free T4 results are associated with a variety of temporary and chronic thyroid conditions. Low free T4 results in conjunction with a low TSH level or high free T4 results along with a high TSH may indicate a pituitary gland condition.
Interpretation:
| TSH | FREE T4 | FREE OR TOTAL T3 | PROBABLE INTERPRETATION |
|---|---|---|---|
| High | Normal | Normal | Mild (subclinical) hypothyroidism |
| High | Low | Low or normal | Hypothyroidism |
| Low | Normal | Normal | Mild (subclinical) hyperthyroidism |
| Low | High or normal | High or normal | Hyperthyroidism |
| Low | Low or normal | Low or normal | Non-thyroidal illness; rare pituitary (secondary) hypothyroidism |
| Normal | High | High | Thyroid hormone resistance syndrome (a mutation in the thyroid hormone receptor decreases thyroid hormone function) |
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| A woman does a thyroid neck check in the mirror. Picture credit: |
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