Thyroxine (T4)

  • 08-03-2019

Other names: Free Thyroxine, free T4, FT4, Total Thyroxine, TT4

The thyroid gland is located in front of the lower neck. Despite its relatively small size, it produces hormones that affect every cell, tissue, and organ in the human body. The thyroid gland produces two types of hormones: thyroxine (T4) and triiodothyronine (T3). T4 contains 4 atoms of iodine and T3 contains 3 iodine atoms (T4 loses an atom of iodine while converting to T3). Thyroid hormones regulate energy metabolism, weight, body temperature, and even mood.

Interpret now "Complete Blood Count (CBC)"
Interpret now "Comprehensive Metabolic Panel (CMP)"
Interpret now "Urinalysis (UA)"

The most common cause of thyroid disorders worldwide is iodine deficiency. In iodine-sufficient areas, most people with thyroid disorders have autoimmune diseases.

In order to avoid iodine deficiency, there are various international programs, for example, the mandatory use of iodized salt in the industrial production of bread. One of the universal methods for the prevention of iodine deficiency conditions (in particular endemic goiter), approved by the World Health Organization, is the use of iodized salt in food. In more than 140 countries, this is an effective way to combat iodine deficiency. In addition, the American Academy of Pediatrics (AAP) recommends pregnant and breastfeeding women take a daily supplement with 150 mcg of iodine to reach a total of 290 mcg per day.

Over a century of research has confirmed the relationship between thyroid hormones and the pathophysiology of various types of cancer. A large body of evidence suggests that subclinical and clinical hyperthyroidism increase the risk of several serious malignant neoplasms, while hypothyroidism can reduce aggressiveness or delay the onset of cancer.

A thyroxine (T4) test is performed to evaluate thyroid function and diagnose diseases. T4 exists in the body in two forms:

  • free T4 (not associated with blood proteins, freely travels into body tissues)
  • bound T4 (bound to proteins, does not enter the body tissues)

Total thyroxine (total T4) measures free and bound T4. Free T4 (FT4) is the active form of thyroxine and is not affected by protein levels. Free T4 is considered a more accurate test than total T4 for testing thyroid function.

Units of measure for thyroxine (T4)

Total thyroxine (total T4) is typically measured in nmol/L (nanomole/liter), or mcg/dL, (μg/dl, microgram/deciliter).

Conversion factor: nmol/L x 0.077688 = mcg/dL, mcg/dL x 12.872 = nmol/L

Free thyroxine (free T4) is measured in ng/dl (nanograms per decilitre), or pmol/L (picomole/liter).

Conversion factor: pmol/L x 0.077688 = ng/dl, ng/dl x 12.872 = pmol/l

Reference Range for Thyroxine (T4)

Reference intervals may vary among different laboratories.

Normal range for total T4 in adults: 5-11 mcg/dL (57-148 nmol/L)

Normal range for free T4 in adults: 0.7-2 ng/dL (9-25 pmol/L)

Reference ranges for total T4 and free T4 varies in children according to age and during pregnancy.

Reference Values For Thyroxine in Pregnancy

Total thyroxine (T4 total) can be elevated during pregnancy by an average of 1.5 times due to the hormone estrogen.

Normally, in the first trimester of pregnancy, TSH may be low, and the level of free T4 may be increased. In the second and third trimesters, a reduced amount of free T4 can be determined. It should be borne in mind that during pregnancy, thyroid function is monitored by the level of thyroid-stimulating hormone (TSH), and an isolated low free T4 has no diagnostic value.

What do T4 test results mean?

The T4 test is usually ordered when TSH levels are out of the reference range. It's more important to determine the thyroid-stimulating hormone (TSH) level in the body because it is the TSH level that is highly sensitive to the levels of circulating thyroid hormone in our body. Abnormal TSH levels are detected earlier than those of T4. If the TSH levels are within the reference values, then thyroid disorders can be almost completely excluded.

High T4 levels

A slight increase in the free T4 level can be caused by strenuous physical activity or severe stress the day before.

Elevated T4 levels may indicate hyperthyroidism. It can also be a sign of other thyroid problems, such as thyroiditis or toxic goiter.

Hyperthyroidism is a condition in which the thyroid gland produces too many hormones. Symptoms include irritability, nervousness, muscle weakness, unexplained weight loss, sleep disturbances, and heart palpitations.

Low T4 Levels

Abnormally low T4 levels may indicate hypothyroidism.

Hypothyroidism is a condition in which the thyroid gland does not produce enough hormones. With a lack of thyroxine, the work of all the systems in your body is disrupted. Symptoms include weakness, fatigue, depression, memory problems, weight gain, irregular periods.

A 2012 study showed that 2-4% of women in the reproductive age have low levels of thyroid hormones. Hypothyroidism can affect fertility due to anovulatory cycles, luteal phase defects, hyperprolactinemia, and an imbalance of sex hormones.

Thyroid Function Tests (TFTs) Interpretation