Thyroxine (T4) is the main hormone produced by the thyroid gland. About 80% of the hormone produced by the thyroid gland is T4. T4 is then converted to the more potent thyroid hormone triiodothyronine (T3) by the liver, kidney and other cells in the body. T4 and T3 together regulate the basal metabolic rate in our body, body temperature, heart rate as well as mood and sleep.

However, almost all the T4 released into the bloodstream is bound to proteins such as albumin and thereby remains inactive. Only a very small proportion of unbound free T4 (FT4) is biologically active.

The FT4 test is often performed together with a thyroid-stimulating hormone (TSH) test and sometimes also a free triiodothyronine (FT3) test to evaluate thyroid function. Since only the unbound free form of the hormone is biologically active, it is important to measure FT4 and not just total T4.

In short, it is useful to analyze FT4 levels in the blood to:

• Detect an underactive thyroid (hypothyroid)

• Detect an overactive thyroid (hyperthyroid)

The reference range for FT4 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 FT4 values.

High free thyroxine (FT4) 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 thyroxine (FT4) 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

Levothyroxine, which is a medication commonly prescribed to people diagnosed with hypothyroidism, is a synthetic form of T4. The FT4 test does not differentiate between synthetic and natural T4. 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.

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