T4 (free): the norm. The hormone T4 (free): a blood test for thyroid hormones

Of Great importance in the normal functioning of the thyroid gland is hypothalamic-pituitary thyroid system. It provides control over the synthesis, production and activity of hormones of the thyroid gland.

General information

The Hypothalamus secretes TRF (tireur-pin-releasing factor). He, in turn, stimulates the secretion and synthesis of thyroid stimulating hormone (thyrotropin – TSH). TSH participates in processes associated with other steroids. In particular, it stimulates the secretion, accumulation, metabolism and synthesis of triiodothyronine (T3) and thyroxine (T4). More than 99% of these two steroids circulating in the blood bound to transport proteins form. Less than one percent remains in the free form. The level of unbound steroid in most people is interconnected with the functional state of the thyroid gland.

Properties of thyroxine

Hormone T4 (free) contributes to regulation of normal development and growth, ensuring the maintenance of body temperature and thus preserving heat generation. The connection has an impact on all phases of the metabolism of carbohydrates, partly on vitamin and lipid metabolism. The hormone T4 (free) is an important component of development in the fetal and neonatal periods. The concentration of the compounds indicates the clinical state of thyroid status, since changes in the level of total thyroxine can be triggered by the irregular functioning of the thyroid gland or changes in the number of transport proteins. During the day the maximum content of the steroid is determined 8 to 12 hours, and the minimum – from 23 to 3. During the year, higher levels of T4 (free) reaches from September to February, minimum – during summer. In the antenatal period (during pregnancy), the concentration of thyroxine is increased gradually reaching a maximum level by the third trimester. Throughout life the content of thyroxine in humans regardless of gender remains relatively constant. The decrease in hormone levels observed after forty years.

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What can be said Of the level of thyroxine?

If T4 (free) is clearly elevated, it is considered a confirmation of hyperthyroidism. Reduced concentration indicates hypothyroidism. The independence of the steroid from thyroxine binding globulin gives you the opportunity to use it as a reliable diagnostic test. This is especially important for conditions that are accompanied by changes in the level therocksandiego globulin. These include oral contraceptives (oral), pregnancy, receiving androgens or estrogens. Changes characteristic of persons with hereditary predisposition to increase or decrease in the concentration of globulin. To donate blood for free T4 is recommended for the diagnosis of secondary hypothyroidism, due to abnormalities in the hypothalamic-pituitary level. In this case, the TSH is not changed or is increased. As a rule, elevated levels of thyroxine could also be a high level of serum bilirubin, obesity, and a tourniquet when it seems to a blood test. T4 (free) is not changed by a serious disease unrelated to thyroid. The level of total thyroxine may be reduced.

Training for laboratory research

A month before the blood donation rule out hormonal therapy (if no specific instructions from the endocrinologist). For two or three days prior to analysis stop the use of iodine-containing drugs. Blood should be taken to the x-ray studies use of contrast agents. On the eve of blood donation have to refrain from physical activity, to eliminate stress. Before the study, half an hour, you should calm down, allow the breath to normal. The analysis is carried out on an empty stomach. With the last meal must be not less than eight hours (but preferably 12 hours). Not allowed to drink coffee, juice or tea. You can only water.

Low levels of thyroxine

T4 (free) (norm for women and men – 9-19 mkmol/liter) may be reduced in the postoperative period, in secondary hypothyroidism (inflammation of the pituitary gland, thyrotropinoma, the Sheehan's syndrome). The reduction of the concentration is also noted on the basis of reception of anabolic steroids, thyreostatics, anticonvulsants medicine and drugs lithium. The reduced level of thyroxine in the use of oral contraceptives, octreotide, methadone, clofibrate. Possible causes include a diet with limited amount of protein, deficiency of iodine, the use of heroin, contact with lead. T4 (free) may be reduced in the tertiary (inflammation of the hypothalamus, head trauma), acquired, congenital hypothyroidism (on the background of extensive resection, and tumors of the thyroid, autoimmune thyroiditis, endemic goiter).

Increasing concentration

The Level of this hormone, as T4 (free) (the norm in women and men mentioned above) may be elevated in cases of toxic goiter, TSH-independent hyperthyroidism, obesity. Increasing the concentration noted on the background of nephrotic syndrome, therapy with heparin. The reasons also include choriocarcinoma, taking thyroxin on the basis of hypothyroidism,postnatal changes in the thyroid gland, chronic liver disease. T4 (free) can be increased in the syndrome of resistance to thyroid steroids, genetic disallowances of hyperthyroxinemia, state, provoking a decrease in the concentration of tyrosinekinase globulin.

Thyroxine and pregnancy

Thyroid Hormones are involved in almost all processes in the body. As mentioned above, compounds regulate metabolism, influence the activity of other steroids. Pathology of the thyroid gland can be accompanied by increase and decrease of its functions. The special value of violations in the activities of the authority are in the period of carrying a child. Changes in the functional state of the thyroid gland affects the course of pregnancy, her character, the outcome and the condition of the newborn. Rarely a bearing is expressed in endocrine pathologies. The diseases of this kind usually lead to reproductive disorders, infertility. Often pregnancy is diagnosed goiter (enlargement of thyroid diffuse) respecting euthyroid, as well as autoimmune thyroiditis, provoking changes in the hormonal background. For prenatal period is characterized by changes in the functional state of the gland.

Recommendations

For the correct interpretation of laboratory results during pregnancy should take into account some points. Determination of total T3 and T4 is not useful. This is because during pregnancy their concentration is increased in half. In the study of unbound levels of thyroxine concentration should be determined in conjunction with the TSH. Thyroxine increased slightly at about 2% of pregnant women. In the first half of the prenatal period, there is a normal decrease in TSH (approximately 20-30% of patients with singleton and all in multiple pregnancies). In the treatment of thyrotoxicosis studied only T4 (free). The level of unbound thyroxine in the later stages could be borderline low. The concentration of TSH will remain within the normal range.