The thyroid gland regulates the work of almost all human organs and systems. Its function can be determined with a blood test for thyroid hormones and TSH.
A blood test for thyroid hormones and TSH – determination in the blood of the concentration of the main thyroid hormones (T4, T3), as well as thyroid stimulating hormone of the pituitary gland (TSH). The analysis reveals a dysfunction of the thyroid gland: hypothyroidism (weakened work) and hyperthyroidism (increased work of the gland). TSH is produced by the pituitary gland and stimulates the thyroid gland in a negative feedback manner. If you need to increase the production of thyroid hormones, the TSH in the blood rises. As soon as the concentration of T4, T3 in the blood exceeds the required level, the TSH level decreases. A blood test for TSH is a screening (primary) examination of the function of the thyroid gland, as it reflects the slightest disturbance.
Thyroxine (T4) and triiodothyronine (T3) are formed directly by the thyroid gland and are involved in the regulation of metabolism: they accelerate the burning of fats and carbohydrates, increase body temperature, increase heart rate, and so on. T3 is mainly formed from T4 by the cleavage of one iodine atom, T3 is more active.
The work of the thyroid gland is not easy to regulate; under the influence of various reasons, various links of this complex mechanism can fail. Therefore, the list of studied hormones is determined in each specific case individually.
The material for analysis is venous blood. The term of execution is usually 1-2 days. It is possible to use rapid tests, which usually have a higher cost.
When is an analysis for thyroid hormones and TSH prescribed?
- with suspicion of hypothyroidism , hyperthyroidism;
- when diagnosing other thyroid pathology;
- after thyroid surgery;
- to monitor the effectiveness of conservative treatment of the thyroid gland;
- routine examination of women during pregnancy, as well as examination of persons living in endemic areas with iodine deficiency;
- screening examination of a healthy population to detect hypothyroidism – determination of TSH.
Preparing for a thyroid hormone and TSH test
Blood for analysis is taken strictly in the morning on an empty stomach (fasting interval 6-8 hours), since the daily level of hormone concentration varies greatly (in the morning it rises, in the evening it decreases). In advance, you should discuss with your doctor the need to interrupt drug treatment, if any, before the examination.
Normal values of thyroid hormones in the blood test
The results of analyzes in different laboratories can vary significantly. This is due to the use of different reagents and techniques. Therefore, it is better to take a repeated blood test for thyroid hormones and TSH in one institution and take into account the norms that a specific laboratory indicates.
Standard values of thyroid hormones and TSH (RIA – radioimmunoassay, ELISA – enzyme-linked immunosorbent assay):
Thyroid stimulating hormone (TSH)
- RIA 0.6–3.8 μIU / ml;
- ELISA 0.24-2.9 μIU / ml.
Total thyroxine (total T4)
- RIA 62-141 nmol / l;
- ELISA 65–160 nmmol / l.
Total triiodothyronine (total T3)
- RIA 1.17–2.18 nmol / l;
- ELISA 1.04-2.50 nmol / l.
Free thyroxine (free T4, not bound to blood proteins)
- RIA 1.5-2.9 μg / 100 ml;
- ELISA 10-25 pmol / l.
Free triiodothyronine (free T3, not bound to blood proteins)
- RIA 0.4 ng / 100ml;
- ELISA 4–8 pmol / l.
Interpretation of results
The human body has the highest compensatory capabilities, and even a severe pathology of the thyroid gland can proceed subclinically for a long time (against the background of normal health). Therefore, the results of a blood test for thyroid hormones are an important diagnostic criterion even in the absence of complaints.
An increase in the concentration of hormones T3, T4 in the blood indicates a hyperfunction of the thyroid gland (hyperthyroidism), a decrease – about hypothyroidism. The concentration of TSH allows you to find out where the cause of the disease lies, whether the pathology of the thyroid gland is primary or secondary. Often, for further diagnostics, a more extensive definition of the function of the thyroid gland is required: an analysis for thyroglobulin (TG) – a protein-precursor of thyroid hormones, antibodies to it (AT-TG) or to thyroid peroxidase (AT-TPO), a test for thyroxine binding globulin (TSH) , which inactivates T3, T4 in blood plasma, etc.
Blood test for thyroid hormones during pregnancy
For a period of up to 15 weeks, all the needs of the fetus for thyroid hormones are provided by the mother’s body. Lack of thyroid hormones during this period can disrupt the development of the unborn baby. Therefore, in women at risk for the development of thyroid pathology, it is very important to determine its function even before pregnancy or in the early stages.
The thyroid gland in the first half of pregnancy normally increases the production of its hormones. At the same time, the TSH level is slightly reduced, and T4 may exceed the upper limit of the norm. By the end of pregnancy, hormones should return to general levels.