FT3 test

What Is the FT3 (Triiodothyronine) Test?

FT3 is one of the main thyroid hormones. Its function is essential for the proper functioning of the body.

The hormones FT3 (triiodothyronine) and FT4 (thyroxine) are produced by the thyroid gland. They regulate the activity of most organs, support metabolic processes, and influence the functioning of the central nervous system. The thyroid produces mainly (about 85%) thyroxine (FT4). Triiodothyronine is also produced by the thyroid, but around 80% of FT3 comes from the conversion of thyroxine, which takes place in the cells of the body. The remaining portion is secreted directly by the thyroid.

FT3 levels are measured mainly for the detailed diagnosis of thyroid diseases.

The primary indication for the test is suspected hyperthyroidism, especially when low TSH is accompanied by normal FT4 levels. Measuring triiodothyronine is not recommended for adjusting thyroxine dosage in patients with hypothyroidism.

Measuring FT4 levels is considered more reliable than measuring FT3, as triiodothyronine levels depend largely on non-thyroidal factors.

The test involves drawing blood from a vein, which is then sent to the laboratory for analysis.

FT3 Function

Functions of the triiodothyronine (FT3) hormone:

• regulation of metabolism
• supporting the development of the nervous system in newborns
• supporting the cardiovascular system

Thyroid hormones are essential for proper body function, which is why diagnosing suspected thyroid disorders is extremely important.

What Does the Test Diagnose?

Elevated FT3 levels together with TSH below the normal range may indicate hyperthyroidism. Low FT3 with elevated TSH may indicate hypothyroidism.

When Should You Test FT3?

Laboratory testing of thyroid hormone levels is recommended in individuals with suspected hypothyroidism or hyperthyroidism.

The presence of thyroid disease depends on hormone concentrations. Abnormal conversion of FT4 to FT3 may be influenced by factors such as:

• poor diet (deficiency of iron, zinc, selenium, vitamins A, B, and B12)
• weight gain or weight loss
• diabetes
• excess levels of mercury, lead, or pesticides in the body
• alcohol abuse
• surgical procedures
• infections
• mental health disorders
• heart failure

Hormonal imbalance may lead to excess FT4 relative to FT3, and impaired conversion of FT4 to FT3 can result in symptoms of hypothyroidism.

How to Prepare for an FT3 Test?

A small amount of water may be consumed immediately before the FT3 test.

It is recommended to have the blood drawn in the morning hours (7:00–10:00).

• The test involves drawing blood according to the standard procedure (skin disinfection, application of a tourniquet, and applying pressure to the puncture site — after the blood draw, the puncture site should be pressed for 5 minutes with the arm extended).
• The blood draw itself lasts from a few to several seconds.
• No special preparation is required for the test.
• Blood collection should preferably take place in the morning.

FT3 Test Results

If the results are abnormal, it is recommended to consult a doctor for proper interpretation.

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Reference ranges (may vary between laboratories)

• FT3: usually 2.25–6.0 pmol/l (≈ 1.5–4 ng/l).
• TSH: typically 0.4–4.0 µIU/ml.

How to interpret results in practice?

Test results should always be interpreted together with TSH levels. The combined assessment of FT3 and TSH is crucial, as normal FT3 may coexist with abnormal TSH. This pattern is often characteristic of subclinical hyperthyroidism or hypothyroidism.

Factors That May Affect Test Results

• poor diet,
• alcohol,
• infections.

Who Should You Consult About Abnormal Results?

• Endocrinologist
• Internist