Free thyroxine (fT4) and free triiodothyronine (fT3) tests

When should you have your fT4 and fT3 levels tested?

If your TSH test result is above, below or at the limit of the reference range, your doctor will certainly order further investigations, most commonly a free thyroxine (fT4) test and, less often, a free triiodothyronine (fT3) test as well. The result report is issued in Polish.

Thyroxine is a hormone produced by the thyroid gland in response to thyroid-stimulating hormone (TSH). Thyroxine (T4), in turn, activates triiodothyronine (T3) in the cells. Triiodothyronine is the active form of thyroid hormone, a hormone that stimulates metabolism. When sufficient amounts of T3 and T4 are present in the body, TSH activity is inhibited.

Most thyroid hormones are bound to proteins (inactive). As protein concentrations can be affected by various factors, the following should be measured:

• fT4– free thyroxine,
• fT3– free triiodothyronine.

These tests are largely independent of protein concentrations, so they are recommended more often than tests for total thyroxine and triiodothyronine, which include the protein-bound forms. The reference ranges for fT3 and fT4 change during pregnancy. Many laboratories have lowered the upper reference limit for fT4 in women in trimester III of pregnancy.

We start by measuring TSH levels:

• Measuring TSH is the most sensitive test for detecting abnormal thyroid function. The test is so sensitive that it reveals even thyroid disorders without symptoms, including those where fT3 and fT4 are within the reference range.
• In hyperthyroidism (excessive hormone production), the thyroid usually produces hormones without TSH stimulation, so TSH levels are low.
• In hypothyroidism (insufficient hormone production), the opposite occurs: TSH levels are high.
• A disadvantage of the test is that TSH levels change relatively slowly. Repeating tests at short intervals, especially just after a change in treatment, is therefore pointless: the result will reflect the situation a month ago rather than the current situation.

Low TSH levels

• To confirm thyroid disease (usually hyperthyroidism) in people with low TSH levels, we measure the free thyroid hormones: fT3 and/or fT4. In hyperthyroidism, these hormone levels are usually raised.
• These tests are less sensitive, but they also have advantages: for example, FT3 and FT4 levels change more quickly in response to factors such as a change in medication. Measuring just one of these hormones is often sufficient. Your doctor decides which one.
• Subclinical (symptom-free) hyperthyroidism also exists: in this case, TSH levels are low, while the other thyroid hormones remain within the reference range.

Normal TSH levels

• Measuring other hormones when TSH is normal is usually unnecessary.
• There are, of course, exceptions to this rule arising from the limitations described above, such as the delayed response (slowly changing hormone levels) during a sudden flare-up of disease, or the presence of factors outside the thyroid that affect TSH levels.

However, if we have measured FT3 or FT4 despite normal TSH and the absence of factors affecting TSH levels, these results should, of course, be normal.

If they are not, this most often means that the FT3/FT4 measurements have been affected by factors outside the thyroid

• FT3: slightly raised or borderline high levels sometimes occur in moderate iodine deficiency. This is accompanied by TSH levels that are normal but also at the upper limit of the reference range.
• Conversely, low, borderline FT3 levels with low, borderline TSH levels occur in people treated with thyroid hormones, often at high doses.

High TSH levels

• These usually indicate hypothyroidism.
• If the patient’s symptoms raise doubts about the diagnosis of hypothyroidism, we should measure FT4.
• If there is no doubt about the diagnosis, measuring FT4 is sometimes omitted because it is a less sensitive test, and when TSH is only slightly raised, FT4 is usually within the reference range (subclinical hypothyroidism).

In summary: when should you have your fT4 and fT3 levels tested?

Testing is usually recommended after an abnormal TSH level (too high or too low) has been found. Too much T4 may indicate excessive hormone production by the thyroid gland (hyperthyroidism). Too little T4, on the other hand, may mean that the thyroid gland is not producing enough hormones (hypothyroidism). Measuring free thyroxine (fT4) also allows assessment of a patient with an existing goitre, that is, an enlarged thyroid gland. Sometimes, measuring thyroxine levels in the blood is helpful when investigating infertility in women.

* If your results fall outside the reference range, we recommend consulting a doctor.

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