Thyroxine (T4)

Are you always tired and cold, or does your heart race and are you losing weight for no reason? The T4 test measures thyroxine, the main hormone produced by the thyroid, and alongside TSH shows whether the thyroid is underactive or overactive. From a single blood collection, you can find out how much hormone your thyroid produces.

Who the test is intended for

• people with symptoms that may suggest an underactive thyroid: fatigue, sensitivity to cold, weight gain, dry skin or constipation;
• people with symptoms that may suggest overactivity: palpitations, restlessness, sweating, weight loss or hand tremor;
• when TSH is outside the reference interval;
• people with an enlarged thyroid or a lump in the neck;
• women with irregular periods or difficulty becoming pregnant;
• for monitoring treatment of thyroid disease, when the doctor requests total hormones or wants to compare the report with previous ones.

Save with a panel

Test Thyroxine (T4) is also included in these panels.

Thyroid health panelTSH, T3, T4Price 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 ›

All panels including Thyroxine (T4) (5) ›

What the test reveals

Thyroxine makes up around 90 % of the hormones the thyroid releases into the blood. The thyroid secretes it in response to TSH from the pituitary gland, and in tissues T4 is converted into the more active T3. Thyroid hormones affect energy use, heart function, breathing, digestion and the menstrual cycle.

The test measures total T4, bound and free together. Almost all T4 travels in the blood bound to carrier proteins, so their quantity also affects the report. Today, free thyroxine, fT4, is therefore measured more often alongside TSH, while total T4 is used when the doctor requests it or when you are comparing the report with previous ones.

What the test involves and how to prepare

The sample is blood from a vein, and collection takes a few minutes. You do not need to fast for thyroid tests unless blood glucose or lipids are also being measured. 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. Do not stop treatment yourself.
• If you take biotin (for example, hair and nail supplements), state this at blood collection, as it can affect thyroid test results.
• At blood collection, also state if you are pregnant or take contraceptive pills or hormone replacement therapy.

What your result means

The reference interval depends on the method and is printed on the report; different limits apply during pregnancy. The doctor interprets the report, always alongside TSH, symptoms and the medicines you take.

• Low T4 with elevated TSH means an underactive thyroid (hypothyroidism), most often due to Hashimoto’s thyroiditis.
• Elevated T4 with low TSH suggests an overactive thyroid (hyperthyroidism), for example due to Graves’ disease, nodules that independently produce hormones or thyroid inflammation.
• Elevated T4 with normal TSH most often results from a greater amount of carrier proteins, for example during pregnancy or with the pill, rather than thyroid disease. The doctor may then request fT4.
• Low T4 with normal TSH may accompany a lack of carrier proteins, severe illness or some medicines. Less often, the cause is in the pituitary, so the doctor decides on further investigation.

In people taking levothyroxine, the doctor adjusts the dose primarily according to TSH. The cause of a disorder is shown by the antibodies anti-TPO and anti-TG and ultrasound, as assessed by the doctor.

How to arrange testing

You can have blood collected without an appointment or referral at one of our branches across Croatia (listed on the Contact page). Prices for all tests and panels are available in the price list. TSH, T3 and T4 from the same sample are included in the Thyroid health panel.

Frequently asked questions

What is the difference between T4 and fT4?

T4 is total thyroxine, while fT4 is only its free portion, which acts in cells. Total T4 also depends on carrier proteins, so fT4 is more reliable when their amount changes.

Why is my T4 elevated if I take the pill?

Oestrogen in the pill increases the amount of proteins carrying thyroid hormones, so total T4 rises even though the thyroid functions normally. If TSH is normal, this is usually not a sign of disease.

Is T4 enough to check the thyroid?

No. The first and most sensitive indicator of thyroid function is TSH, and T4 or fT4 are interpreted alongside it.

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. Take other medicines as usual unless your doctor tells you otherwise.