Thyroid T4 and FT4: Difference between T4 and FT4

What is bound and unbound T4?

The thyroid gland is a butterfly-shaped structure present in the base of your throat, below Adam’s apple and above the collar bones. This gland is responsible for the production of hormones like T3 or triiodothyronine and T4 or thyroxine. In this test level of the thyroxine (T4) hormone measurement is done by collecting a blood sample.

Thyroid T4

This test is also known by the names like total T4 concentration, thyroxine screen, free T4 concentration, and FT4. The thyroid gland synthesizes triiodothyronine or T3 and thyroxine or T4 in response to the thyroid stimulating hormone (TSH). The pituitary gland present in the brain takes the ownership of synthesis of TSH.

T4 is present in two forms in the body which includes free T4 or unbound T4 and bound T4. Free T4 floats freely in the bloodstream and reaches the tissues which use T4. The remaining bound T4 gets attach with the proteins which prevent its entry into the tissues.

Approximately more than 99% of T4 is present in the bound form. Since T4 conversion occurs into T3 and free T4 hence, T4 becomes a more important hormone to measure for its levels in the body. Any changes in the T4 will certainly show up first indicating any thyroid disorder.

The free T4 test is also termed as FT4 or Free T4 Index.  Individuals with hyperthyroidism will show an elevated FT4 or FTI level, whereas an opposite happens in case of hypothyroidism individuals.

Also, combining the TSH test along with the FT4 or FTI gives an accurate idea about thyroid gland functioning.

T3 and T4 mainly help in controlling the storage and utilization of the energy or metabolism in the body.

These thyroid hormones also help in controlling and managing several other body processes like:

  • Breathing
  • Menstruation
  • Functioning of the heart
  • Nervous system functioning
  • Body temperature and its regulation
  • Cholesterol levels
  • Body weight management
  • Brain development
  • Skin moisture and healthiness

This T4 and FT4 test help in revealing if your thyroid gland is overactive or you are suffering from hyperthyroidism condition and if it is under-active or you are suffering from hypothyroidism condition.

When should you go for Thyroid T4 and FT4 test?

If you observe any symptoms of thyroid problems than you must go for T4 and FT4 Tests without any delay.

Symptoms of hyperthyroidism, or overactive thyroid activity, are as follows:

  • Irritability, mood swings, and anxiety
  • Arms and legs weakness because of muscle weakening
  • Sleeping troubles like sleep apnea etc.
  • Irregular heartbeats or palpitations
  • Hand tremors
  • Excessive sweating

Thyroid T4

  • Sensitivity towards heat
  • Extreme fatigue or tiredness
  • Sudden weight loss which is unexplained
  • Frequent bowel movements than normal
  • Eye troubles like irritation or even bulging, these are mainly because of Graves’ disease and is a common cause of hyperthyroidism
  • Menstrual irregularities
  • Breast enlargement and erection problems in men

Symptoms of hypothyroidism, or inactive thyroid activity, are as follows:

  • Extreme tiredness
  • Sensitivity towards cold
  • Weight gain even after controlling diet and physical activity
  • Hair thinning and even hair loss
  • Eye swelling or puffiness
  • Slower heart rate
  • Shortness of breath (SOB)

Thyroid T4

  • Constipation
  • Menstrual irregularities

Are there any other tests available for thyroid?

Your physician might also recommend getting few other tests, done to measure thyroid-related substances —

which includes:

  • TSH, T3, and free T3 levels altogether give a wider coverage of thyroid issues
  • Imaging or scans of the thyroid gland
  • Radioactive iodine uptake scanning is done of the thyroid gland or RAIU scan
  • Thyroglobulin, which is used in the formation and storage of thyroid hormones
  • TSH receptor-stimulator antibodies, which helps in the diagnosis of Graves disease
  • Thyroid anti-peroxidase antibodies and thyroglobulin antibodies, which helps in the diagnosis of Hashimoto’s thyroiditis

What are the test procedure and its results?

The T4 and Free T4 tests require a blood sample collection, which is done through a needle from a vein in your arm.

Insertion of the small needle just causes a prick and does not involve any major risks. Sometimes, though a patient might experience bruising, bleeding, infection, and even dizzy feeling. Afterward, the pricking site might be slightly sore but will eventually get better on its own.

The test results, measured in micrograms per deciliter (mcg/dL). The normal range for total T4 including both free and bound T4 levels might vary from lab to lab, its normal recommended range is between 4.6 to 11.2 mcg/dL.

Free T4 can be, measured in following two ways:

  • This with normal ranges, is identifiable by the testing method which the lab uses mostly
  • Free T4 index, a formula based analysis which utilizes total T4 and thyroid hormone-binding index. The normal range for the free T4 index is between 1.1 to 4.3 mcg/dL.

If your results appear as high total T4 or a high free T4 index, it indicates you might be experiencing from hyperthyroidism. If your results show a low total T4 or a low free T4 index, it indicates a presence of hypothyroidism condition.

Several health conditions might influence the test results causing false results. So, prior taking the test you must discuss your disease history with your physician. In order to avoid any false test results and wrong diagnosis of disease


T4 tests, done to identify the thyroid troubles. Since T4 exists in two forms in the body so two different tests, done for thyroid evaluation.

  1. T4 bounded with proteins prevents T4 entrance into several tissues which are in need of thyroid hormone. An imbalance of its levels reveals the disorder.
  2. Free T4, which enters several targeted tissues to show its impact. So this free T4 test is an important way of determining the thyroid functioning.

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