What are the other Names for this Test? (Equivalent Terms)
- T4 Blood Test
- Total Tetraiodothyronine Blood Test
- Total Thyroxine Blood Test
What is Total T4 Blood Test? (Background Information)
- Thyroxine (T4) is a hormone produced by the thyroid gland. It stimulates nearly every cell in the body to increase oxygen and energy consumption. T4 increases body metabolism, causing increased growth and development
- The thyroid gland is situated in the front of the throat, just below the cartilage of the Adam’s apple. Apart from T4, the thyroid gland produces two other hormones namely, T3 or triiodothyronine, and calcitonin
- The pituitary gland stimulates the thyroid gland, through production of thyroid-stimulating hormone (TSH). TSH production is in turn stimulated by thyrotropin-releasing hormone (TRH), from the hypothalamus of the brain
- The system operates via a negative feedback loop; TSH stimulates thyroid gland hormone production, whose increase then inhibits TRH production by the brain. This results in decreased stimulation of TSH production, leading to decreased thyroid gland hormones
- Calcitonin is made by C cells. It plays a role in decreasing calcium levels in blood, by stimulating calcium excretion by the kidneys, and through calcium uptake by the bones
- The difference between T3 and T4 is that T3 is 4-5 times more effective than T4 at increasing cellular metabolism. However, nearly 90% of T3 that reaches the cells, is converted from T4
- Thus, in other words, the thyroid gland produces larger quantities of T4 relative to T3 - roughly in a 10:1 ratio. But, T3 is primarily responsible for causing cell growth and development
- The reason for this conversion step is to add an opportunity for the body to regulate and fine-tune its metabolic rate. If the rate of T4 to T3 conversion is decreased, cellular metabolism can be lowered, without having to synthesize more hormones
- T3 and T4 are carried by binding proteins. Only, less than 1% of T3 and T4 circulate freely in blood. These binding proteins include:
- Thyroid-binding globulins (TBGs), which carry 70% of T4 and 75% of T3
- Transthyretin, also known as thyroid-binding prealbumin (TBPA), and
- Albumin, a plasma protein
- Thyroid hormones bound to proteins, serve as the body’s reserves. At any time, more than 1 week’s supply of T3 and T4 are bound by proteins
- As cells take up free hormones, bound hormones are released and become free. This is because of the equilibrium that exists between free and bound hormones, which adds yet another opportunity for regulation
- The result of this equilibrium between free and bound thyroid hormones is that levels of free T3 and free T4 directly relate to levels of bound and total T3 and T4
- As their names suggest, T3 and T4 are made using iodine. Without iodine, the production of these hormones stop, or is reduced. This can lead to the formation of a goiter (swelling) in the neck as levels of fluid and hormone precursors accumulate
- The Total T4 Blood Test is a test to assess the levels of both bound and free T4 in blood. This test is affected by the levels of thyroid binding proteins. It is used to aid in the diagnosis of thyroid disorders. It is also used to monitor thyroid therapy
What are the Clinical Indications for performing the Total T4 Blood Test?
Following are the clinical indications for performing a Total T4 Blood Test:
- Following up to TSH and free T3 tests
- Early or late onset of puberty
- Distinguishing between primary and secondary hypogonadism
- Masculinization in females, such as excessive hair growth
- Decreased sex drive
- Erectile dysfunction
- Diminished menstrual cycle
- Abnormal uterine bleeding
How is the Specimen Collected for Total T4 Blood Test?
Following is the specimen collection process for Total T4 Blood Test:
Sample required: Blood
Process: Insertion of a needle into an arm vein.
Preparation required: No special preparation is needed prior to the test.
What is the Significance of the Total T4 Blood Test Result?
The significance of Total T4 Blood Test is explained:
- Increased total T4 may indicate:
- Acute psychiatric illness
- Excessive intake of iodine
- Hyperemesis gravidarum
- Thyrotoxicosis factitia
- Graves’ disease
- Decreased total T4 may indicate:
- Conditions with decreased TBGs
The laboratory test results are NOT to be interpreted as results of a "stand-alone" test. The test results have to be interpreted after correlating with suitable clinical findings and additional supplemental tests/information. Your healthcare providers will explain the meaning of your tests results, based on the overall clinical scenario.
Additional and Relevant Useful Information:
- Certain factors interfere with the results of the Total T4 Blood Test. These include altitude, pregnancy, age, strenuous exercise, and nutrition status
Certain medications that you may be currently taking may influence the outcome of the test. Hence, it is important to inform your healthcare provider, the complete list of medications (including any herbal supplements) you are currently taking. This will help the healthcare provider interpret your test results more accurately and avoid unnecessary chances of a misdiagnosis.