T4 (Thyroxine), Total

Description: A T4 Total test is a blood test that measures thyroxine free levels in your blood’s serum to evaluate your thyroid’s health and to screen for, diagnose, and monitor thyroid disorders such as hypothyroidism.

Also Known As: Total T4 Test, Total Thyroxine Test, T4 Test, T4, T4 Total Test, Bound T4 Test

Collection Method: Blood Draw

Specimen Type: Serum

Test Preparation: No preparation required

When is a T4 Total test ordered?

When a person exhibits symptoms of hyperthyroidism or hypothyroidism, a Total T4 test may be administered, especially if a TSH test result is abnormal.

Hyperthyroidism can cause the following signs and symptoms:

Hypothyroidism can cause the following symptoms:

When a person is being treated for a thyroid problem, Total T4 testing, along with other thyroid tests, may be requested on a regular basis.

Thyroid testing will most likely be ordered early and late in the pregnancy, as well as for a period after delivery, to monitor the mother and baby in pregnant women with thyroid abnormalities.

In the United States, thyroid hormone screening is routinely performed on babies as part of newborn screening programs.

What does a T4 Total blood test check for?

The thyroid gland, a small butterfly-shaped structure that lays on the windpipe towards the bottom of the throat, produces one of two primary hormones: thyroxine. Triiodothyronine is the other primary thyroid hormone, and together they help govern the rate at which the body utilizes energy. T4 in the blood is almost entirely linked to protein. The remaining portion is free and is the hormone's biologically active form. This test determines how much Bound T4 is present in the blood.

A feedback loop controls T4 production. The hypothalamus releases thyrotropin releasing hormone, which encourages the pituitary gland to generate and release thyroid-stimulating hormone when the amount of T4 in the blood drops. The thyroid gland is thus stimulated to produce and/or release more T4 as a result of TSH. TSH release is blocked as T4 content in the blood rises.

T4 accounts for over 90% of thyroid hormones. The thyroid gland releases accumulated T4 into circulation when the body requires it. T4 is either free or bound to protein in the blood. The amount of free T4 in the body is just about 0.1 percent of total T4. In the liver or other tissues, T4 is converted to T3. T3, like T4, is mainly attached to protein, however the physiologically active forms of T3 and T4 are the free versions. Free T3 in circulation is 4 to 5 times more active than free T4.

Dry skin, weight gain, cold intolerance, weariness, and irregular menstruation are among signs of hypothyroidism that occur when the thyroid gland does not produce enough T4 due to thyroid malfunction or insufficient TSH. Myxedema, or severe untreated hypothyroidism, can cause heart failure, convulsions, and coma. Hypothyroidism in children can slow growth and sexual development.

When the thyroid gland generates too much T4, the rate of a person's body functions increases, resulting in hyperthyroidism symptoms such as anxiety, increased heart rate, difficulty sleeping, weight loss, puffiness and dry itchy eyes, and hand tremors.

The most prevalent causes of thyroid dysfunction are connected to autoimmune illnesses. Hyperthyroidism is caused by Graves disease, while hypothyroidism is caused by Hashimoto thyroiditis. Thyroiditis, thyroid malignancy, and excessive or insufficient TSH production can all induce hyperthyroidism or hypothyroidism. By measuring Total T4, the influence of these variables on thyroid hormone synthesis can be recognized and monitored.

Note: Free T4 Index (T7) will only be calculated and reported if test code code 861 (T3 Uptake) is ordered as well.

Lab tests often ordered with a T4 Total test:

Conditions where a T4 Total test is recommended:

How does my health care provider use a T4 Total test?

Total thyroxine tests are used to assess thyroid function and detect thyroid disorders, such as hyperthyroidism and hypothyroidism, after the thyroid stimulating hormone level has been found to be abnormal.

The thyroid gland produces T4 and another hormone called triiodothyronine. They aid in the regulation of the rate at which the body expends energy and are governed by a feedback system. TSH promotes the thyroid gland's synthesis and release of T4 and T3.

The majority of T4 and T3 in the blood is attached to protein, while just a small amount is free. Total T4, free T4, total T3, and free T3 can all be measured in blood testing. The total T4 test has been around for a long time, but it is influenced by the quantity of protein in the blood that can bind to the hormone. The active form of thyroxine, free T4, is unaffected by protein levels. Many people believe that the free T4 test is a more accurate reflection of thyroid hormone activity, and it has largely supplanted the total T4 test.

A Total T4 test can be used in conjunction with or after a TSH test, and occasionally with a free T3 test to:

In the United States, babies are routinely tested for T4 and TSH levels to rule out congenital hypothyroidism, which can lead to mental retardation if left untreated.

Thyroid abnormalities can sometimes be detected using Total T4 and TSH, however professional opinions differ on who should be screened and when they should start.

Thyroid antibodies, as well as a Total T4 test, may be ordered if a health practitioner suspects someone has an autoimmune-related thyroid problem.

What does my T4 Total result mean?

In general, high total T4 levels suggest an overactive thyroid gland, while low total T4 levels suggest an underactive thyroid gland. The test results are not diagnostic in and of themselves, but they will urge the health care provider to conduct additional testing to determine the reason of the excess or deficiency.

A range of temporary and chronic thyroid disorders are linked to both decreased and increased total T4 levels. A pituitary gland issue could be indicated by low total T4 levels along with a low TSH level, or by high total T4 levels combined with a high TSH.

When thyroid tests are done to monitor treatment for thyroid or pituitary diseases, the results will tell the doctor whether the treatment is working and/or if a dose adjustment is required. People with hyperthyroidism, for example, have their total T4, total T3, and TSH levels examined on a regular basis while taking anti-thyroid medicines to ensure that the drugs are effective and to reduce doses if thyroid hormone levels fall too low. TSH and total T4 levels are monitored on a frequent basis in hypothyroid patients to ensure that the correct dose of thyroid hormone is being given to bring TSH levels back to normal.

We advise having your results reviewed by a licensed medical healthcare professional for proper interpretation of your results.