Thyroxine (T4) is one of two hormones produced by the thyroid gland. It works to speed up the rate of your metabolism. Most T4 is bound to carrier proteins in the blood - it is only the free, or unbound, T4 that is active in the body, which is measured in this test. Free T4 is the less active of the two main thyroid hormones. To have an impact on your cells it needs to convert to the more active T3 when your body needs it.
What might a low result mean?
Low free T4 may indicate that the thyroid is underactive (hypothyroidism) and is struggling to produce enough thyroid hormones. A low FT4 result means that there is less thyroid hormone available to convert to the active T3, resulting in a slower metabolism. There are numerous symptoms of an underactive thyroid - including sensitivity to cold, gaining weight and low mood and depression.
Free T4 levels may also be slightly lower in pregnant women due to an increase in the amount of the carrier protein, thyroid binding globulin. This means that more T4 is bound, leaving less "free" to be biologically active in your cells.
What might a high result mean?
Raised free T4 can indicate an overactive thyroid. High levels of FT4 tend to lead to excess of the other thyroid hormone, T3, resulting in an overactive metabolism. Symptoms of an overactive thyroid can include sensitivity to heat, difficulty in gaining weight, a rapid heartbeat and feelings of nervous anxiety.
Sometimes raised FT4 can be seen if you have already been diagnosed with a thyroid disorder and are taking too much thyroid hormone replacement (thyroxine).