Blood test for T4

T4 hormone or thyroxine is one of the main endocrine hormones produced by the thyroid gland.

This enzyme consists of four iodine atoms and helps to normalize the energy balance in the body, and also activates protein synthesis.

The main part of T4 freely moves around the human body. The so-called free thyroxine, not associated with proteins, plays an important role in catabolic processes and general metabolism.

The protein-bound remaining thyroxine enters the bloodstream. Bound and free fractions of T4 in total determine the total thyroxine.

What is T4, its main functions

Thyroxine is one of the main enzymes of the thyroid gland. The concentration of such a hormone in the body as a whole determines the quality and stable operation of the thyroid gland. It should be noted that T4 is far from the most active hormone of the endocrine system. For example, triiodothyronine, the main active metabolite of T4, is more than ten times more active than its predecessor.

T3 and T4 are also often referred to as thyroid hormones, as they are produced by the thyroid gland, which in Latin sounds like “thyroid”.

T4 is an iodine-containing thyroid hormone that has four iodine atoms and two amino acid residues tyrosine in its chemical composition. Other names for this hormone are thyroxine and tetraiodothyronine.

The name T4 is directly related to the number of iodine atoms in its composition. Its structure is so simple that the concentration of this substance in the body is quite easy to determine in the laboratory, and this also makes it possible to synthesize T4 artificially, which is often used in hormone therapy.

Although this hormone is not more active than its metabolite, it can reflect changes in the state of the thyroid gland more clearly. The content of four iodine atoms in the thyroxine molecule allows the thyroid gland to work smoothly and stably.

In the blood, the hormone T4 is in a bound form. During the synthesis of thyroxine in the thyroid follicles, it is captured by a protein - thyroxine-binding globulin or TSH. Thanks to this capture, T4 is transported to various internal organs and body systems. It is free thyroid hormones that have the main effect on the human body: free T3 and free T4. Their content can give a clear picture of the presence of various pathologies of the endocrine system or inflammatory processes in the thyroid gland.

The main functions of thyroxine in the body are:

  • normalization of the nervous system;
  • getting energy from fats and glycogens ;
  • normalization of heart rate;
  • normalization of heart rate;
  • ensuring cellular respiration, saturation of cells and tissues of the body with oxygen.

An excess of this hormone in the body is most often characterized by a rapid heartbeat or tachycardia. increased irritability and nervousness, a sharp causeless weight loss.

Blood test for T4 and how to prepare for it

Most often in endocrinological practice, a complex analysis for T3, T4 and TSH is prescribed. Such hormonal screening allows you to accurately and most accurately determine the various pathologies of the thyroid gland and the endocrine system.

But in some cases, at the discretion of the attending physician, separate studies may be prescribed. In this case, the main indications for the appointment of a study on T4 are the period of pregnancy, suspicion of thyroid pathology, and the use of thyreostatic drugs.


T3 and T4 hormones are best studied in free forms. Evaluation of the amount of hormones produced is very important in hormonal therapy with thyreostatics, since in this case there is often a sharp decrease in the level of T3 or T4.

At the same time, TSH levels decrease at a slower rate. The results of the analysis in such cases are considered to be unreliable.

If a patient is treated for a long time for thyroid insufficiency, it is necessary to undergo an examination for the concentration of endocrine hormones in a systematic manner.

In this way, it is possible to determine how successful the prescribed therapy is. But in this case, a separate analysis for TSH is usually prescribed, and a study for free T4 is prescribed for certain medical reasons.

When taking a blood test for T4, it must be remembered that taking thyroxine preparations gives distorted results, since their amount will be attached to the amount of thyroxine produced by the thyroid gland.

During pregnancy, T4 levels may also be elevated, as thyrotropin concentrations decrease due to the action of hCG produced by the placenta.

Therefore, it is not enough for pregnant women to pass individual tests, a general analysis for T3, T4 and TSH is usually prescribed.

The use of anabolic steroid drugs, carbamazepine, methadone, phenytoin, thyreostatics, octreotide and clofibrate contributes to a decrease in T4 concentration.

Levothyroxine, aspirin, tamoxifen, propranolol, amiodarone, furosemide, propylthiouracil, and danazol increase T4 levels.

In order for the results of a blood test for thyroid hormones to be most reliable before donation, it is necessary to adhere to the following rules:

  • refuse to eat for two to three hours before the analysis, or take it in the morning on an empty stomach;
  • exclude emotional and physical activity approximately one day before blood donation;
  • in agreement with the attending physician, stop using hormonal drugs, including contraceptives;
  • stop using nicotine three hours before the start of the analysis.

T4 reference values ​​and norms

Independent interpretation of the results of the analysis will not give the desired result. It should be carried out by the attending physician, who is based not only on the norms of the hormone content, but also on other individual indicators of the patient.

The norms of the hormone T4 in different laboratories can vary significantly, as different equipment and certain reagents are used for the study.

For an adult, the rate of free thyroxine in the blood usually ranges from 10.8 to 22.0 pmol / liter, for a child 12.5 - 21.5 pmol / liter. The indicators of a pregnant woman vary in the range from 7.6 to 18.6 pmol / liter. The norm of total thyroxine for a man is 59-139 nmol / liter, for a woman - 71-142 nmol / liter, for children from five years old - 83-172 nmol / liter. In pregnant women, this figure is 75-230 nmol / liter.

T4 indicators in the female body

The T4 hormone plays a very important role specifically for the female body, since it significantly affects the reproductive function. With the normalization of the hormonal background of thyroid hormones in the body of a woman, eggs are formed correctly, and ovulation occurs.

The T4 hormone also controls the following processes in a woman's body:

  • body weight regulation;
  • elimination of harmful cholesterol ;
  • the rate of metabolic processes;
  • prevents thrombosis.

During pregnancy, the presence of this hormone plays an important role in the laying of the internal organs and systems of the embryo, and also contributes to the proper development of the fetus. In the first trimester, with the help of thyroxin, the nervous system of the embryo is formed. And its quantitative indicators influence the further normalization of its development.

That is why the production of T4 in pregnant women increases significantly, and its deficiency in the blood is associated with the transfer of part of the enzymes to the child's body. Lack of T4 in pregnant women leads to various pathological changes in the embryo, and in some cases to spontaneous abortion or miscarriage.

As menopause approaches, the concentrations of this hormone in the female body decrease. In addition, there are changes in the amount of T4 depending on the seasons or time of day. In the morning, its level is the highest, in the second half of the day it decreases significantly, and at night, the concentration of thyroxin in the blood is minimal. In the autumn-winter period, an increase in the level of this hormone is also clearly expressed, and in the summer its concentration decreases.

An increase in the level of T4 in the body

An increase in this hormone in the body, not during pregnancy, can be caused by the following reasons:

  • obese;
  • glomerulonephritis;
  • artificial preparations of thyroxin;
  • HIV;
  • myeloma;
  • thyroiditis in the acute phase;
  • postpartum thyroid disorders.

At high concentrations of thyroid hormone T4 in the human body, some characteristic symptoms are observed. Among them are heart rhythm disturbances, tachycardia, increased irascibility and irritability, increased sweating, tremor of the upper and lower extremities. weight loss, increased fatigue, loss of strength. An overestimation of T4 values ​​often contributes to the acceleration of the breakdown of fats and leads to an excess of energy, which negatively affects various internal organs, complicating their work and accelerating their wear and tear.

Such a state also negatively affects the nervous system, which entails frequent mood swings, negative emotional coloring, tremors throughout the body.

With an increase in the level of thyroxin in the human body for a fairly long period of time, calcium is washed out of the bone tissue, which leads to osteoporosis and frequent fractures.

A decrease in the concentration of T4 in the body

A decrease in the concentration of thyroid hormone T4 most often signals the development of hypothyroidism - insufficiency of the thyroid gland.

The reasons for which this pathology may occur are as follows:

  • traumatic brain injury;
  • treatment of thyrotoxicosis with radioactive iodine or artificial thyroxine;
  • endemic goiter, congenital or acquired;
  • autoimmune thyroiditis;
  • inflammation or tumors in the pituitary and hypothalamus;
  • thyroid surgery;
  • Sheehan's syndrome, characterized by the death of pituitary cells.

If the patient does not have any of the above pathologies, an additional test for the TSH hormone is prescribed. Often the concentration of free T4 in the primary analysis is underestimated. In such cases, a re-analysis is prescribed. Insufficiency of the function of the thyroid gland is not cured, it is lifelong.

The therapy in this case is the use of artificially synthesized thyroxin, the dosage of which is chosen by the doctor endocrinologist on an individual basis.

Summing up

A blood test for the T4 hormone allows you to identify various pathological processes occurring in the thyroid gland and the endocrine system. Its excess, as well as its deficiency, entails negative and unnecessary consequences, therefore it is extremely necessary, if there is the slightest suspicion of problems with the thyroid gland, to undergo hormonal studies. Based on the results of the analysis, a qualified endocrinologist will make the correct diagnosis and prescribe the appropriate therapy, which will quickly and easily get rid of the existing pathology.