fT4

Are you tired and always cold, or, conversely, sweating, experiencing a racing heart and losing weight? fT4 (free thyroxine) shows how much hormone your thyroid actually supplies to the body. Alongside TSH, it is a basic test for assessing thyroid function and monitoring treatment.

Who is this test for

• people with symptoms of an underactive thyroid: fatigue, sensitivity to cold, weight gain, dry skin or constipation;
• people with symptoms of overactivity: restlessness, palpitations, weight loss, sweating or hand tremor;
• when TSH is elevated or low in the report;
• people taking levothyroxine or medicines that reduce thyroid activity, to monitor the dose;
• women planning pregnancy or who are pregnant and have thyroid disease or increased risk;
• in pituitary disease, when TSH alone is not reliable.

What the test reveals

The thyroid produces mainly thyroxine (T4). Almost all T4 in the blood is bound to carrier proteins, and only a small free portion enters cells and acts. fT4 measures precisely this portion, so pregnancy and contraceptive pills affect it less than total T4.

The pituitary gland directs thyroid function using TSH. fT4 is therefore interpreted together with TSH: TSH shows whether the body is asking for more or less hormone, while fT4 shows how much the thyroid actually supplies. When overactivity is suspected, the doctor often also requests fT3.

Save with a panel

Test FT4 is also included in these panels.

Thyroid panel with free hormone fractionsTSH, fT3, fT4Price of individual tests: 30.00€27.00€Details ›Complete thyroid panelTSH, T3, T4, fT3, fT4 and 2Price of individual tests: 102.00€92.00€Details ›Hormonal status panel – womenTSH, fT3, fT4, cortisol and oestrogen, progesterone and 3113.00€Details ›

All panels including FT4 (13) ›

The test and preparation

Blood is taken from a vein, and collection takes a few minutes. You do not need to fast unless other tests requiring it are being performed at the same time. It is best to come in the morning and have follow-up tests at a similar time of day.

If you take levothyroxine, take your tablet after blood collection on the testing day, because a tablet taken immediately beforehand may temporarily raise fT4. If you take biotin (for example, hair and nail supplements), state this at blood collection, as it can affect thyroid test results.

What your results mean

In adults, fT4 is usually approximately 12 to 22 pmol/L, but limits depend on the method and differ during pregnancy, so those printed on your report apply. The doctor interprets the report alongside TSH, symptoms and the medicines you take.

• Low fT4 with elevated TSH: an underactive thyroid (hypothyroidism), most often due to Hashimoto’s thyroiditis. The cause can be supported by anti-TPO.
• Normal fT4 with elevated TSH: subclinical hypothyroidism. The doctor usually repeats the tests after several weeks to three months.
• High fT4 with low TSH: an overactive thyroid (hyperthyroidism), for example in Graves’ disease, nodules or thyroid inflammation. In people taking levothyroxine, it may indicate too high a dose.
• Low fT4 with low or normal TSH: a less common combination that may suggest pituitary disease and requires further investigation.

How to arrange testing

You can come for blood collection without an appointment or referral at one of our branches throughout Croatia (listed on the Contact pagepage). Prices for all tests and panels are listed in the price list. Because fT4 is interpreted alongside TSH, a more practical option is the Thyroid Panel with Free Hormone Fractions (TSH, fT3 and fT4), and for an initial comprehensive investigation, the Complete thyroid panel.

Frequently asked questions

What is the difference between fT4 and T4?

T4 measures all thyroxine in the blood, bound and free, so it also changes when the amount of carrier proteins changes, for example during pregnancy or with contraceptive pills. fT4 measures only the free, active portion and is therefore measured more often today.

Is testing TSH alone enough?

As a first step in people without pronounced symptoms, often yes. fT4 is added when TSH is abnormal, with clear symptoms, in pregnancy, in pituitary disease and for monitoring treatment. The order of tests is described in the article Laboratory investigation of the thyroid.

May I take my thyroid tablet before blood collection?

On the testing day, take your levothyroxine tablet after blood collection so the result can be compared with previous ones. Do not stop treatment for days before the test unless your doctor explicitly tells you to.

What does slightly elevated fT4 with normal TSH mean?

Most often, it results from a levothyroxine tablet taken before blood collection, biotin or a temporary change during an acute illness. Less often, there is interference with the method or a rare disease, so the doctor repeats the test before drawing a conclusion.