The hypothalamic-pituitary thyroid system is of great importance in the normal functioning of the thyroid gland. It provides control over the synthesis, production and activity of thyroid hormones.
General information
The hypothalamus secretes TRF (thyrotropin-releasing factor). It, in turn, stimulates the release and synthesis of thyroid-stimulating hormone (thyrotropin - TSH). TSH is involved 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 circulate in the blood in the form associated with transport proteins. Less than one percent remains in free form. The level of unbound steroid in most people is correlated with the functional state of the thyroid gland.
Properties of thyroxine
The hormone T4 (free) contributes to the regulation of normal development and growth, ensuring the maintenance of body temperature and thus maintaining heat generation. Connection rendersinfluence on all stages of carbohydrate metabolism, in part - on vitamin and lipid metabolism. The hormone T4 (free) is an important component of development in the prenatal and neonatal periods. The concentration of the compound indicates the clinical state of the thyroid status, since a change in the level of total thyroxin can be triggered by disturbances in the activity of the thyroid gland or a change in the number of transport proteins. During the day, the maximum content of the steroid is determined from 8 to 12 hours, and the minimum - from 23 to 3. During the year, the highest level of T4 (free) reaches from September to February, the minimum - in summer. In the prenatal period (during pregnancy), the concentration of thyroxin increases, gradually reaching a maximum level by the third trimester. Throughout life, the content of thyroxine in people, regardless of gender, remains relatively constant. A decrease in hormone levels is noted after forty years.
What can the level of thyroxin tell about?
If T4 (free) is clearly elevated, then this is considered a confirmation of hyperthyroidism. Decreased concentration indicates hypothyroidism. The independence of the steroid content from thyroxin-binding globulin makes it possible to use it as a reliable diagnostic test. This is especially important in conditions that are accompanied by a change in the level of thyroxin-binding globulin. These include taking contraceptives (oral), pregnancy, receiving androgens or estrogens. Changes are also characteristic of persons with hereditarypredisposition to increase or decrease in the concentration of globulin. Donating blood for free T4 is recommended for the diagnosis of secondary type hypothyroidism caused by pathologies at the hypothalamic-pituitary level. In this case, the content of TSH does not change or increases. As a rule, an increase in the content of thyroxine can be caused by a high level of bilirubin in the serum, obesity, as well as the application of a tourniquet when a blood test is taken. T4 (free) does not change in severe diseases not related to the activity of the thyroid gland. At the same time, the level of total thyroxine may decrease.
Preparation for laboratory research
One month before donating blood, hormones are excluded (unless there are special instructions from the endocrinologist). Two or three days before the analysis, the use of iodine-containing drugs is stopped. Blood should be donated before X-ray studies using contrast agents. On the eve of blood donation, you need to refrain from physical exertion, eliminate stressful situations. Before the study, half an hour, you should calm down, bring your breathing back to normal. The analysis is carried out on an empty stomach. At least eight hours should pass from the last meal (but preferably 12 hours). It is not allowed to drink coffee, juice or tea. Only water is allowed.
Decreased thyroxine level
T4 (free) (the norm for women and men is 9-19 pmol / liter) may decrease in the postoperative period, with secondary hypothyroidism (inflammation of the pituitary gland, thyrotropinoma, syndromeSheehan). A decrease in concentration is also noted due to the intake of anabolics, thyreostatics, anticonvulsants, and lithium preparations. The level of thyroxine decreases with the use of oral contraceptives, octreotide, methadone, clofibrate. Other possible causes include a diet with a limited amount of protein, lack of iodine, heroin use, exposure to lead. T4 (free) may decrease with tertiary (inflammation of the hypothalamus, TBI), acquired, congenital hypothyroidism (against the background of extensive resection and tumor of the thyroid gland, autoimmune thyroiditis, endemic goiter).
Increase concentration
The level of a hormone such as T4 (free) (the norm for women and men is indicated above) can be increased with toxic goiter, TSH-independent thyrotoxicosis, and obesity. An increase in concentration is observed against the background of nephrotic syndrome, heparin therapy. The reasons also include choriocarcinoma, thyroxine intake due to hypothyroidism, postpartum changes in the activity of the thyroid gland, and chronic liver damage. T4 (free) may be elevated in thyroid steroid resistance syndrome, genetic disalbunemic hyperthyroxinemia, conditions that provoke a decrease in the concentration of thyroxine-binding globulin.
Thyroxin and pregnancy
Thyroid hormones are involved in almost all processes in the body. As mentioned above, the compounds regulate metabolic processes, affect the activity of other steroids. Pathologies of the thyroid gland can be accompanied by both an increase anddecrease in its function. Violations in the activities of the body are of particular importance during the period of bearing a child. Changes in the functional state of the thyroid gland affect the course of pregnancy, its nature, outcome and the condition of the newborn. Rarely, gestation occurs with severe endocrine pathologies. Diseases of this kind, as a rule, lead to violations of the reproductive function, infertility. Most often during pregnancy, goiter is diagnosed (an increase in the thyroid gland of a diffuse nature) with the preservation of euthyroidism, as well as autoimmune thyroiditis, which provokes changes in the hormonal background. The prenatal period is characterized by a change in the functional state of the gland.
Recommendations
In order to correctly interpret the results of laboratory tests during pregnancy, several points should be taken into account. The definition of total T3 and T4 is not informative. This is due to the fact that during pregnancy their concentration is increased by one and a half times. When examining the level of unbound thyroxine, its concentration should be determined in combination with the content of TSH. Thyroxine is slightly increased in 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 a singleton and all with multiple pregnancies). In the treatment of thyrotoxicosis, only T4 (free) is examined. The level of unbound thyroxine in the later stages can be marginally lowered. At the same time, the concentration of TSH will remain within the normal range.
