Free Thyroxine Index

What is the Free Thyroxine Index (FTI)?

The free T4 index (FTI) is a blood test used to diagnose thyroid disorders. T4, also called thyroxine, is a thyroid hormone. The test measures how much of it is in your blood to help determine whether your thyroid gland is underactive (hypothyroidism) or overactive (hyperthyroidism).

The Free Thyroxine Index (FTI) is a laboratory test used to assess thyroid function. The FTI provides an estimate of the amount of free, unbound T4 hormone in the bloodstream. This is important because only the unbound T4 is biologically active and can affect various body functions.

Doctors use the FTI to diagnose and monitor thyroid disorders, such as hyperthyroidism and hypothyroidism. Abnormal FTI levels can indicate an overactive or underactive thyroid gland, which can have a significant impact on metabolism, energy levels, and overall health. 

Therefore, the Free Thyroxine Index is a valuable tool in assessing thyroid function and guiding appropriate treatment decisions.

If your healthcare provider orders an FTI, it will probably be performed along with other thyroid tests, including thyroid-stimulating hormone (TSH) and triiodothyronine (T3). The results are then analyzed together to help with a diagnosis.

The FTI and other thyroid tests are performed on blood samples, which are fairly quick and simple to obtain and very low risk.

Currently, direct measurement of serum free thyroxine is starting to replace the FTI formula in most clinical situations. Autoantibodies to thyroid hormones can, however, interfere with the assay.

Patients are sent for an FTI and other thyroid tests if they have symptoms that could indicate a thyroid disease, especially in the presence of other risk factors (female gender, age below 40, family history of thyroid disorders).

How is the Free Thyroxine Index calculated?

The FTI  is calculated using a formula that involves total thyroxine (T4) and a measurement called the thyroid hormone-binding index (TBI). 

FTI = Thyroxine (T4) / Thyroid Binding Capacity

FTI levels in healthy adults:

In healthy individuals, the FTI remains relatively constant and helps compensate for abnormal levels of binding proteins. The normal range for the FTI is typically between 1.2 - 4.9 Units, though specific reference ranges may vary depending on the laboratory and population being tested.

What is the thyroid and how does it relate to T4?

The thyroid is located in the front of your throat and is shaped like a butterfly. Its purpose is to make hormones and regulate several important bodily functions, such as energy use, weight, body temperature, and mood.

In your body, T4 functions in two different forms. 

→ One form bonds with a protein to perform certain tasks, 

→ and another form doesn't, which allows it to do different jobs.

The FTI test checks only for the "free" form, which is the form not bonded with a protein. It's useful for evaluating thyroid function.

Is the FTI a separate blood test?

The FTI is not itself a separate blood test, but is calculated from the results of the total T4 test and the T3 resin uptake test. Because the FTI is often made inaccurate by medical conditions that change blood protein levels, it is not used very much in clinical medicine today. Instead, the free T4 level is now measured directly.