Blood test for thyroid hormones, norm and pathology. Explanation of analyzes.- Causes, symptoms and treatment. MF.

It is necessary to give blood test for thyroid hormones on an empty stomach( you can not even drink).Blood is taken from the vein. It is advisable to donate blood until 10 - 10.30.On the instructions of the doctor before the study to exclude the use of drugs of thyroid hormones. The day before the test is taken, it is possible to exclude exercise and stress, if possible, half an hour before blood collection, it is preferably in a calm state.

Indicators that are noted for the analysis and evaluation of the thyroid gland: T3( triiodothyronine) total and free, T4( thyroxine) total and free, TSH( thyroid-stimulating hormone of the pituitary gland), antibodies to thyroglobulin, antibodies to thyroxine peroxidase), calcitonin.

Timely treatment of diseases of the thyroid gland will help to avoid serious consequences and surgical interventions. An early diagnosis of hypothyroidism is urgently needed, since its neglected form( hypothyroid coma) can lead to irreversible consequences. Diagnosis of the expressed forms of hypothyroidism does not cause any special difficulties. It is more difficult to identify mild forms with not always typical symptoms, especially in elderly patients. Where it is easy to suspect cardiovascular failure. Diseases of the kidneys, etc. Diagnosis of hypothyroidism is specified by a number of diagnostic laboratory blood tests for the content of iodine and thyroid hormones( T3, T4, thyreotropic).Functional deficiency of the gland is characterized by a decrease in iodine and T3, T4, as well as an increase in thyroid-stimulating hormone.

Laboratory diagnostics of the thyroid gland pathology

The thyroid gland examination solves for today one of the most urgent problems of medicine: early detection of impaired gland function for effective therapy with the least consequences, both the treatment itself and pathophysiological disorders in the body.
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thyroid hormone determination •
thyroid pathology markers •
regulatory thyroid hormones are used to evaluate thyroid function in the laboratory. Only the physician will select the optimal examination scheme to obtain sufficient information for diagnosis and control of the thyroidstatus of the patient.

Thyroid-stimulating hormone

Thyroid-stimulating hormone( TTG) is a hypophyseal hormone that, acting on the thyroid gland, plays a major role in ensuring a normal level of circulation of iodothyronines, T3 and T4.The level of TSH is controlled by the hypothalamic hormone TRH( thyrotropin-releasing hormone) and is inversely related to T3 concentrations.
With primary hypothyroidism, when the production of thyroid hormones is reduced, the level of TSH is usually kept high. On the other hand, with secondary or tertiary hypothyroidism, when the decrease in the production of thyroid hormones is due to impaired functions of the pituitary and hypothalamus, the TSH level is usually low. With hyperthyroidism, the level of TSH is usually lower( cases of secondary hyperthyroidism are extremely rare).

Third Generation TTG

Like the previous test, it is intended for the determination of TSH.But unlike it has an order of magnitude greater sensitivity, which makes it possible to determine very low concentrations of TSH with high accuracy.allowing to identify subclinical forms of the disease, as well as to conduct more close monitoring of the therapy.

General T4

Thyroxine( T4) , the main thyroid hormone, circulates normally in an amount of about 58 to 161 nmol / L( 4.5 to 12.5 μg / dL), most of it is bound to transport proteins, mainly TSG, the state. Against the background of a normal level of proteins that bind thyroid hormones, hyperthyroidism is characterized by an increased, and hypothyroidism - a lowered level of circulating T4.However, in patients with abnormal levels of proteins that bind thyroid hormones, such a parallel between the concentration of total T4 and thyroid status is excluded.
Because the level of common T4 is often out of the norm in people with euthyroid status or may be normal for thyroid disorders, it is desirable to assess the level of circulating TSH, for example, by analyzing T3 Uptake. In case of thyroid dysfunction, the values ​​of total T4 and T3 Uptake will deviate from normal in one direction, whereas with changes in the level of TSH in patients with euthyroid status they will deviate from the norm in opposite directions. The product of T4 and T3 Uptake, divided by 100, is known as the Free Thyroxine Index FT4I.

Free T4

The circulating thyroid hormone thyroxine( T4) is almost entirely bound to transport proteins, the main one being thyroxine-binding globulin( TSG), with balance maintained between them in such a way that a change in the level of transport proteins causes a corresponding change in the level of total T4, while the level of free T4 remains relatively unchanged. Therefore, it can be expected that the concentration of free T4 will correspond more closely to the clinical thyroid status than the concentration of the common T4, since the out-of-normal T4 results may reflect both thyroid dysfunction and simply the( physiological or pathological) change in the level of transport proteins.
Thus, for example, TSG elevation is typical in pregnancy, oral contraceptives and estrogen therapy cause a rise in the level of total T4, often above normal limits, without causing a corresponding rise in the level of free T4.In addition, changes in the level of TSH can sometimes hide thyroid dysfunction, raising the level of total T4 in patients with hypothyroidism or reducing it in patients with hyperthyroidism to normal values. And in these cases, the concentration of free T4 will also more accurately reflect the true thyroid status than the concentration of the common T4.

General T3

Under normal physiological conditions, T3 is approximately 5% of thyroid hormones in serum. Although the T3 concentration is lower than the concentration of circulating T4, it has a higher metabolic activity, a faster turnover, and a greater volume of spread. Reports that, in some cases, thyrotoxicosis, abnormally high concentrations of T3 play a greater role than T4 concentrations, increase the significance of T3 measurement. In addition, the definition of T3 is an important link in the monitoring of patients with hypothyroidism receiving sodium-lyoteronin therapy. Unlike the "T3 Uptake" test, which evaluates the saturation of the thyroid hormone binding proteins, the T3 test actually measures the levels of circulating triiodothyronine. Many reports indicate that there is a clear difference in T3 levels in people with euthyroidism and hyperthyroidism, but the differences between hypothyroidism and euthyroidism are less pronounced.

Many factors not associated with thyroid disease can cause abnormal T3 values. Therefore, the values ​​of total T3 should not be used by themselves in establishing the thyroid status of a particular person. When evaluating the results of the analysis, the serum T4, thyroxine-binding globulin, TSH and other clinical data should be taken into account.

Free T3

Free triiodothyronine is 0.3% of total triiodothyronine in the blood. However, it provides the entire spectrum of metabolic activity and realizes a negative feedback with the pituitary gland. Since the level of cT3 does not depend on the concentration of TSH, its determination accurately characterizes the thyroid status regardless of fluctuations in the content of transport proteins.

Thyroxine-binding globulin

Thyroxin-binding globulin( TSH) is a glycoprotein with a molecular weight of 54,000 daltons, consisting of a single polypeptide chain. It is one of the three protein carriers of thyroid hormones, both thyroxine( T4) and 3,5,3'-triiodothyronine( T3);except for it, the thyroid hormone carriers are thyroxine-binding prealbumin( TSPA) and albumin. Although TSH is present in much smaller amounts than albumin and TSPA, it has a much greater affinity for thyroid hormones and is therefore the main of the binding proteins. In healthy people, only up to 0.05% of the total serum present in T4 is in free( unbound) form. The bound T4 is distributed among the binding proteins as follows: TSH 70-75%, TSPA 15-20%, and albumin 5-10%.

Antibodies to thyreoglobulin

Thyroglobulin is a glycoprotein with a molecular weight of 660,000 daltons, consisting of two subunits, produced only by the thyroid gland. It is the main component of the thyroid colloid and is present in the serum of healthy people. Autoantibodies to thyroglobulin( AT to TG) with the help of sensitive immunoassays are determined in low serum concentrations of 4-27% of healthy people;in higher concentrations, they are determined in 51% of patients with Graves' disease and 97% with Hashimoto's thyroiditis, as well as in 15-30% of patients with differentiated thyroid carcinoma. Measurements of AT to TG have long been used in conjunction with the definition of antibodies to thyroid peroxidase( AT to TPO), helping in the diagnosis of autoimmune thyroid diseases. It is likely that the analysis of AT to TPO as the main test for autoimmune thyroid diseases will replace the combination of AT to TG / AT to TPO due to the higher sensitivity of the AT test to TPO in Graves' disease and Hashimoto's thyroiditis.

It is useful to measure AT to TG in all sera that will be tested for thyroglobulin. Since autoantibodies to thyroglobulin can interfere with both the immunoassay based on competitive binding and the immunometric analysis of thyroglobulin, all patients should perform a sensitive immunoassay for antibodies to thyroglobulin to exclude their effect. The results of thyroglobulin analysis in case of detection of antibodies to thyroglobulin in a patient should not be considered.

Measurements of AT to TG can also provide useful prognostic information in patients undergoing operational treatment for differentiated thyroid carcinoma. If the patient had AT to TG, in the postoperative period, the level of AT to TG in the serum will remain constant or increase with persisting or progressing of the tumor, whereas in patients recognized after prolonged observation of almost cured, the levels of AT to TG generally decrease.

Antibodies to thyroid peroxidase

Antibodies to thyroid peroxidase are autoantibodies to this enzyme. The enzyme thyroid peroxidase catalyzes the process of iodination of tyrosine in thyroglobulin during the biosynthesis of T3 and T4.Until recently, these antibodies were called antimycrosomal( AMA), because they were associated with the microsomal part of the thyroid cells. Modern research has determined that thyroid peroxidase is the main antigenic component of microsomes.

Autoimmune thyroid diseases are the main factor underlying hypothyroidism and hyperthyroidism, and usually develop in genetically predisposed populations. Thus, the measurement of circulating antithyroid antibodies is a marker of genetic predisposition. The presence of antibodies to TPO and an elevated level of TSH make it possible to predict the development of hypothyroidism in the future.

The main autoimmune diseases of the thyroid gland are Hashimoto's thyroiditis and Graves' disease. In fact, in all cases of Hashimoto's disease and in most cases of Graves' disease, antibodies to TPO are increased. High levels of antibodies to TPO in combination with clinical manifestations of hypothyroidism confirm the diagnosis of Hashimoto's disease.

Thyroglobulin

Thyroglobulin is a glycoprotein with a molecular weight of 660,000 daltons, consisting of two subunits, produced only by the thyroid gland. It is the main component of the thyroid colloid and is present in the serum of healthy people. TG is used as a marker of tumors in the thyroid gland, and in patients with a distant thyroid or treated with radioactive iodine, to evaluate the effectiveness of the treatment.