FT4: what it is, what it tells you and when it’s tested

FT4 is free thyroxine: the part of your thyroid’s main hormone that circulates freely in your blood, ready to enter your cells. It is almost always read together with TSH, the hormone your pituitary uses to tell your thyroid to work harder or less.

When it’s tested

Doctors mostly order it when TSH is outside the limits, to see how much your thyroid is really working. They also order it if you have symptoms of a slow or fast thyroid, if you take thyroid hormone, or if there are pituitary problems.

To check the thyroid in people who feel well, TSH is usually enough: it moves first and says a lot on its own. FT4 comes next, to understand what is going on.

That is why Rosa does not test it. Rosa measures TSH, the first test for the thyroid, and explains it together with the rest of the panel.

What it tells you, and what it does not

FT4 shows how much hormone your thyroid is releasing. Read with TSH, it helps show whether your thyroid is working too hard, too little, or whether the problem lies in the pituitary.

High TSH and low FT4: the thyroid is working too little, called hypothyroidism. High TSH and normal FT4: the thyroid is struggling a little but still coping. Low TSH and high FT4: the thyroid is working too hard, called hyperthyroidism.

On its own, though, it does not give the cause. For that you need thyroid antibodies, sometimes an ultrasound, and an examination.

That is why it is read together with other tests:

Normal FT4 levels

The lab decides what is normal, and the unit matters a lot here. FT4 is written in pmol/L, ng/dL or pg/mL, and the same blood gives very different numbers.

A common range in pmol/L is 12 to 22. The same range is about 0.9 to 1.7 in ng/dL, and about 9 to 17 in pg/mL. To go from ng/dL to pg/mL, multiply by ten.

Even in the same unit, each analyser has its own limits, and results from two labs cannot always be compared. Look at the limits printed next to your result, and to follow FT4 over time, stay with the same lab if you can.

In pregnancy the values change and some methods read it less well: your obstetrician uses different limits.

If your FT4 is high or low

What causes high FT4

If FT4 is high and TSH is low, your thyroid is making too much hormone. If TSH is normal instead, the cause is often not the thyroid. The most common causes:

Symptoms of high and low FT4

Often none, if the value is only slightly out of range. When there are symptoms, they come on slowly:

These symptoms are common and have many other causes. FT4 and TSH help show whether your thyroid is involved.

Low FT4: what it can mean

If FT4 is low and TSH is high, your thyroid is working too little: this is hypothyroidism. The most common cause is Hashimoto’s thyroiditis, which shows up in anti-TPO antibodies. It is treated with thyroid hormone, at a dose your doctor sets.

If FT4 is low but TSH is normal or low, the problem may lie in the pituitary, which is not sending the thyroid enough signal. This is rare, but it needs to be understood: your doctor usually refers you to an endocrinologist.

Sometimes no thyroid disease is involved. A serious illness, a hospital stay or some medicines can lower it for a while. In pregnancy FT4 normally falls towards the end.

What can skew the result

A few things change the value without any thyroid problem. It helps to know them before your blood test.

When to worry

Talk to your doctor if:

Frequently asked questions

What are normal FT4 levels? It depends on the unit and the lab. A common range is 12 to 22 pmol/L, which is about 0.9 to 1.7 ng/dL, or 9 to 17 pg/mL. The range on your own report is what counts.

Low FT4 with a normal TSH: what does it mean? Sometimes it is a value just below the limit, which your doctor repeats. Sometimes an illness or some medicines are behind it. More rarely the pituitary is not sending the thyroid enough signal: that is why a doctor should see it.

High FT4 with a normal TSH: what does it mean? Often it is not the thyroid. People on levothyroxine can have a high value in the hours after the tablet. Amiodarone, heparin, biotin or a problem with the test method can also be behind it. Your doctor usually repeats it.

What is the difference between T4 and FT4? Total T4 counts all the hormone, including the part bound to blood proteins. FT4 counts only the free part, the one your cells can use. That is why FT4 is what is usually ordered today.

Does Rosa test FT4? No. Rosa measures TSH, the first test for the thyroid, and LDL cholesterol, which a slow thyroid can raise. If your doctor wants FT4, they order it separately.

Sources