Free Triiodothyronine (FT3)
Thyroid functionFT3

Free Triiodothyronine (FT3)

Free Triiodothyronine (FT3)

May 14, 2021

Thyroid function

What is FT3?

Triiodothyronine (T3) is one of the main hormones produced by the thyroid gland. Although produced in smaller amounts than thyroxine (T4), it is about 4 times more potent. Only a small proportion of T3 found in the blood is produced by the thyroid gland itself, the majority is converted from T4 by the liver, kidney and other cells in the body. T3 and T4 together regulate the basal metabolic rate in our body, body temperature, heart rate as well as mood and sleep.

However, almost all the T3 released into the bloodstream is bound to proteins such as albumin and thereby remain inactive. Only a very small proportion of unbound free T3 (FT3) is biologically active.

For more information about thyroid function and pregnancy specifically, click here.

Why is this analysis important?

The FT3 test is often analysed as a part of a “thyroid profile” together with free thyroxine (FT4) and thyroid stimulating hormone (TSH) test to evaluate thyroid function. Since only the unbound free form of the hormone is biologically active, it is important to measure FT3 and not just total T3.

In short, it is useful to analyse free triiodothyronine (FT3) levels in the blood to:

  • Detect an underactive thyroid (hypothyroidism)

  • Detect an overactive thyroid (hyperthyroidism)


The reference range for free triiodothyronine (FT3) levels in the blood can be different depending on the laboratory and technique used. Doctors usually also take into account a number of factors when evaluating free triiodothyronine (FT3) values.

High free triiodothyronine (FT3) levels in the blood may be associated with:

  • Overactive thyroid (hyperthyroidism)

  • Overmedication with thyroid replacement hormones

  • Insufficient anti-thyroid medication

  • Some benign thyroid nodules

  • Pregnancy

Low free triiodothyronine (FT3) levels in the blood may be associated with:

  • Underactive thyroid (hypothyroidism)

  • Insufficient medication with thyroid replacement hormones

  • Overmedication with anti-thyroid medication

  • Iodine deficiency - essential in the production of thyroid hormones

  • Low conversion of T4 to T3

Other Considerations

Liothyronine, which is a medication sometimes prescribed to people diagnosed with hypothyroidism, is a synthetic form of T3. The FT3 test does not differentiate between synthetic and natural T3. If you are prescribed this medication, you should bring your daily dose to the blood test and take them after the blood is drawn.

Taking high levels of biotin (vitamin B7) supplement can affect the readings from your FT4 test.

Tests that include this marker


Laboratory assessment of thyroid function. Douglas S Ross, MD. UpToDate Nov 07, 2017