Blood Test For Thyroid Hormones: Decoding, Norm

2024 Author: Rachel Wainwright | [email protected]. Last modified: 2024-01-15 19:51

Analysis for thyroid hormones: transcript

The content of the article:

1. Thyroid hormones

2. Indicators determined during the analysis and their normal values

   Thyroid-stimulating hormone

3. Antibodies to thyroperoxidase

   1. Thyroxine and triiodothyronine
   2. Antibodies to thyroglobulin
   3. Antibodies to thyroid-stimulating hormone receptor
4. Indications for a thyroid hormone test?
5. Analysis rules
6. Decoding the analysis for thyroid hormones
7. What you need to know when taking an analysis for thyroid hormones

Thyroid hormone testing is one of the most important and frequently prescribed laboratory tests. The thyroid gland is an organ of the human endocrine system. Its cells produce hormones that take part in regulating the processes of maintaining the internal environment of the body (homeostasis). According to the WHO, about 3% of the world's population suffers from thyroid diseases, and disorders of its functions are noted, according to various authors, in 15–40% of people.

The analysis for thyroid hormones plays an important role in the diagnosis of the endocrine system of the body

Thyroid hormones

A study for thyroid hormones usually includes determining the following indicators:

• thyroxine (T ₄, tetraiodothyronine);
• triiodothyroronine (T ₃);
• thyroid stimulating hormone (TSH);
• antibodies to thyroperoxidase (Ab-TPO);
• antibodies to thyroglobulin (Ab-TG, anti-TG);
• antibodies to the thyroid-stimulating hormone receptor (AT-rTTG).

The main thyroid hormones are thyroxine and triiodothyronine. The rest of the indicators do not belong to them in terms of physiological nature, but are included in the blood test for thyroid hormones, since they play an important role in assessing the functions and state of the endocrine system.

Indicators determined during the analysis and their normal values

Thyroid-stimulating hormone

Thyroid stimulating hormone is secreted by the cells of the pituitary gland - a small gland located in the thickness of the brain substance. The function of thyroid stimulating hormone is to regulate the secretory activity of the thyroid gland, that is, the production of thyroid hormones by its cells. The limits of the norm of this hormone depend on the age of the patient, and in pregnant women, on the trimester of pregnancy.

TSH rate depending on age

Age	TSH level, honey / l
Up to 6 months	0.73 to 4.77
6 months to 14 years	0.7 to 4.17
14 to 19 years old	0.47 to 3.14
Over 19 years old	0.4 to 4

Limit values of TSH levels in pregnant women:

• I trimester - from 0.1 to 2.5 mU / L;
• II trimester - from 0.2 to 3 mU / l;
• III trimester - from 0.3 to 3 mU / l.

An increase in the level of TSH in the blood is observed in the following cases:

• primary hypothyroidism of various origins (hypoplasia or surgical removal of the thyroid gland, iodine deficiency, hereditary violation of the synthesis of thyroid hormones, autoimmune thyroiditis);
• some tumors of the breast or lungs;
• pituitary adenoma;
• tissue resistance to thyroid hormones;
• severe somatic diseases in the recovery stage;
• thyroid cancer.

The reasons for the low level of TSH in the blood can be:

• primary hyperthyroidism due to various reasons (diffuse toxic goiter, toxic nodular goiter, toxic adenoma);
• transient hyperthyroidism;
• overdose of thyroxine preparations;
• hyperthyroidism in pregnant women;
• starvation;
• stress;
• injuries and tumors of the pituitary gland;
• hypothalamic-pituitary insufficiency;
• Itsenko-Cushing's syndrome.

Antibodies to thyroperoxidase

Antibodies to thyroid peroxidase are special immunoglobulins that destroy an enzyme contained in the cells of the thyroid gland and is responsible for the transition of iodine molecules into the active form necessary for the synthesis of thyroid hormones. They are specific markers of thyroid autoimmune damage. Normally, their content in blood ranges from 0 to 34 mU / ml. An increased level of At-TPO is observed in the following cases:

• autoimmune thyroiditis (Hashimoto's disease);
• thyroid cancer;
• diabetes mellitus and some systemic connective tissue diseases (systemic vasculitis, rheumatism, systemic lupus erythematosus).

The detection of an increased level of AT-TPO in pregnant women indicates a high risk of developing hypothyroidism in a child (congenital or developing immediately after birth).

In some cases, more often in women of middle and mature age, an increased level of AT-TPO is observed in people without any diseases.

Thyroxine and triiodothyronine

The main thyroid hormones, as mentioned above, are triiodothyronine (T ₃) and thyroxine (T ₄). When they enter the bloodstream, they very quickly bind to proteins and become inactive. After this complex reaches the target organ, it breaks down and the hormones become active (free) again.

When determining the total T ₃ and T _4, both their bound and free forms are taken into account. At present, preference is given to determining the level of free thyroxine and triiodothyronine, since it has a significantly greater diagnostic value.

Normal values of free T ₄ lie in the range from 9 to 19 pmol / L, and free T ₃ - 2.62 to 5.69 pmol / L.

An increased level of triiodothyronine is observed in hyperthyroidism, thyrotoxicosis, pregnancy, and liver diseases.

The causes of low T ₃ levels can be hypothyroidism, thyroiditis, prolonged fasting.

An increase in thyroxine levels occurs in patients with subacute thyroiditis. Other causes of this condition can be an excess of iodine in the body, pregnancy, and some types of malignant tumors.

Hypothyroidism and prolonged fasting lead to a decrease in the concentration of T ₄ in the blood.

Antibodies to thyroglobulin

Thyroglobulin is a special protein that is a precursor of thyroid hormones. Normally, it does not enter the bloodstream. In some diseases of the thyroid gland (Hashimoto's disease, diffuse toxic goiter), thyroglobudin enters the bloodstream, resulting in the formation of antibodies to it. Normally, the content of these antibodies in the blood should not exceed 115 IU / ml.

Antibodies to thyroid-stimulating hormone receptor

On the membranes of thyrocytes (thyroid cells) there are special structures that can bind to thyroid-stimulating hormone of the pituitary gland. They are called TSH receptors. An increased level of antibodies to them is observed in patients with Graves' disease (diffuse toxic goiter), Hashimoto's disease (autoimmune thyroiditis).

Indications for a thyroid hormone test?

The main indications for the appointment of an analysis for thyroid hormones are the following diseases and conditions;

• significant hair loss;
• sleep disorders (difficulty falling asleep, insomnia, frequent nighttime awakenings);
• decreased mental abilities, memory impairment;
• suspicion of a pituitary tumor;
• losing weight or gaining excess weight for no apparent reason;
• an increase in the size of the thyroid gland and / or its tenderness on palpation;
• diseases of the cardiovascular system (arterial hypertension, arrhythmia);
• liver diseases of a non-infectious nature;
• menstrual irregularities;
• decreased potency;
• male and female infertility;
• systemic autoimmune diseases (dermatitis, lupus erythematosus, rheumatism);
• delayed psychomotor development in children.

In addition, testing for thyroid hormones is sometimes required to assess the effectiveness of the therapy.

Analysis rules

In order for the results of the laboratory study to be as accurate and correct as possible, it is important to follow the rules:

• donate blood in the morning on an empty stomach;
• 24 hours before blood sampling, you must stop smoking, drinking alcohol and significant physical exertion;
• hormonal drugs are canceled one month before the study;
• 72 hours before the analysis, you must stop taking iodine-containing drugs.

The patient should come to the laboratory 20-30 minutes earlier than the appointed time and sit quietly in the hallway. Only after such a short rest, the laboratory assistant proceeds to the procedure for taking blood from the cubital vein for analysis.

For analysis for thyroid hormones, blood is taken from a vein on an empty stomach

Decoding the analysis for thyroid hormones

Only an endocrinologist can correctly interpret the results of a laboratory test for thyroid hormones. At the same time, it takes into account all the indicators, the degree of their deviation from the norm, as well as the clinical signs of a particular disease available in a particular patient.

Changes in the level of T ₃, T ₄ and TSH and its relationship with dysfunction of the thyroid gland

	TSH	T 3	T 4
Hyperthyroidism	Downgraded	Tall	Tall
Primary hypothyroidism	Tall	Decreased or normal	Decreased or normal
Secondary hypothyroidism	Tall	Decreased or normal	Decreased or normal

However, only a specialist can establish what kind of disease underlies the development of a hypothyroid or hyperthyroid state.

What you need to know when taking an analysis for thyroid hormones

The cost of testing for thyroid hormones is quite high. However, if you know some of the rules, then research will be cheaper.

1. AT-TPO level. Thyroid peroxidase antibodies are detected only once. This is due to the fact that the value of this indicator can change without regard to the patient's condition and therefore it is not suitable for assessing the effectiveness of the therapy.
2. General and free T ₃ and T ₄. To diagnose and monitor the effectiveness of treatment, it is sufficient to determine the level of only free or only bound forms of thyroid hormones. It should be borne in mind that more valuable diagnostic tests are free thyroxine and free triiodothyronine. There is no need to define a common and related shape at the same time.
3. Thyroglobulin. It is not necessary to determine this indicator during the initial examination of patients. Indications for its examination are conditions after removal of the thyroid gland, papillary thyroid cancer.
4. At-rTTG. This test is also not prescribed to patients during the initial examination, except in cases where they have signs of severe thyrotoxicosis. It is quite expensive and is necessary exclusively for evaluating the effectiveness of treatment of patients with thyrotoxicosis.
5. Calcitonin is another thyroid hormone whose function is currently not fully understood. It is usually handed over once. Re-examination is indicated only if new nodes appear in the thyroid gland, as well as in patients who have undergone resection or removal of the thyroid gland for medullary cancer.

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Elena Minkina Doctor anesthesiologist-resuscitator About the author

Education: graduated from the Tashkent State Medical Institute, specializing in general medicine in 1991. Repeatedly passed refresher courses.

Work experience: anesthesiologist-resuscitator of the city maternity complex, resuscitator of the hemodialysis department.

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