T4 Test: Purpose, Preparation, Procedure and Understanding the Results

As T4 exists in two forms in your body, there are two kinds of T4 tests which include a total T4 test and a free T4 test.

Updated: September 29, 2019

Thyroid is a small butterfly shaped gland that is located in the lower-front part of your neck that typically produces three hormones namely triiodothyronine (T3), thyroxine (T4), and calcitonin. The thyroid produces T3 and T4 hormones when the pituitary gland releases TSH in the blood. Thyroid-stimulating hormone or TSH is responsible for regulation of metabolism, release of hormones by the thyroid gland, energy generation, heart rate and mood. T4 or thyroxine stimulates our metabolism, energy levels and overall well-being. Both thyroxine (T4) and triiodothyronine (T3) are created and stored in the thyroid.
Most thyroid hormones are attached to a protein within the blood. However, few of them are not bonded to protein in your blood which are known as free T4. This type of T4 are available for use by your body and tissues. As T4 exists in two forms in your body, there are two kinds of T4 tests which include a total T4 test and a free T4 test.
A total T4 test measures the T4 that is bonded to protein along with any free T4 where as a free T4 test measures only the free T4 in your blood. Free T4 is often preferred over a total T4 test as it is available to your body for use and can affect body functions.

Purpose of T4 tests:

When the result of a thyroid-stimulating hormone (TSH) test is abnormal, a T4 test will help your doctor determine what type of problem is affecting your thyroid. The kind of disorders that affect thyroid function include:

  • hyperthyroidism, or overactive thyroid
  • hypothyroidism, or underactive thyroid
  • hypopituitarism, or underactive pituitary gland

One of these conditions is suspected if you have symptoms such as:

  • eye issues, like dryness, irritation, puffiness, and bulging
  • skin dryness or puffiness
  • hair loss
  • hand tremors
  • changes in blood pressure
  • changes in heart rate

Weight changes, difficulty sleeping or insomnia, anxiety, fatigue and weakness, intolerance to cold, sensitivity to light and irregular menstrual are some of the common symptoms you may experience in these disorders.
A TSH and T3 test along with T4 may help your doctor gain a better understanding of your thyroid problem. These tests may also be performed to help assess whether a known thyroid issue is improving.

Purpose of a free T4 Test:

To detect cases of pituitary/hypothalamus insufficiency, a combination of TSH and free T4 test is done. In this case TSH is low, normal or slightly high and the level of free T4 is low.
A free T4 test is also recommended for therapy evaluation with thyreostatic drugs, as low TSH levels are to be expected for months or years after the treatment.

Preparing for a T4 test:

Certain medications such as birth control pills or other medications containing hormones, such as androgens and estrogens, steroids or thyroid-related drugs may affect your T4 levels, especially total T4. Therefore before going for a T4 test your doctor should know about your medications in advance, so that they can advise you to temporarily stop using them or consider their effect when interpreting your results.

Procedure for a T4 test:

Fasting is not required for this test. A band will be tied by a technician around your arm so that the veins of the hand are more prominent to prick the injection. The vein is punctured with a fresh, disposable syringe and the blood is withdrawn after the site is cleaned with an alcohol swab. A cotton swab will be given to you after drawing the blood to place on the place where the injection was pricked. You will be asked to apply pressure on the cotton in order to stop the bleeding. You might notice a bruise in the place where the injection was pricked on your skin. This is normal and should fade away in a few days. But if you experience unbearable pain, or if the area around the puncture becomes red and swollen, immediately follow up with your doctor. These could be signs of an infection.

Understanding the Results of a T4 test:

The total T4 test in adults generally range from 4.5 to 11.2 micrograms per deciliter (μg/dL). For children the result vary based on age and labs. So talk to your doctor about the normal ranges expected for your child.
The free T4 test generally range from 0.9 to 2.4 nanograms per deciliter (ng/dL). Like total T4, free T4 also varies in children according to age.
Because T4 is not the only hormone involved in thyroid function, a normal result on this does not necessarily indicate that the thyroid is functioning properly. There could be still possibility of a thyroid problem. For example, in cases of an overactive thyroid, your T4 results are in normal range but your T3 results are abnormal.

High levels of free T4  :

High levels are seen in cases of hyperthyroidism or overactive thyroid. They may also indicate other thyroid problems, such as thyroiditis or toxic multinodular goiter.
Too much iodine can raise your T4 levels. A recent X-ray involving dye may also raise your T4 test results as X-ray dyes can include iodine. High levels of T4 may also be caused by:

  • high levels of protein in the blood
  • too much thyroid replacement medication
  • too much iodine
  • trophoblastic disease, a group of rare pregnancy-related tumors
  • germ cell tumors

Low levels of  T4:

Low levels are seen in cases of hypothyroidism or underactive thyroid. Some people may have low levels due to circulating antibodies against thyroxine which is very rare. Low levels can also be seen in the last trimester of pregnancy without hypothyroidism being present. Low levels of T4 indicates:

  • a pituitary problem
  • hypothyroidism
  • dietary issues, such as fasting, malnutrition, or an iodine deficiency
  • medications that affect protein levels
  • illness

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