Triiodothyronine (T3) is the most bioactive thyroid hormone. T3 is created when thyroxine (T4) is biotransformed as a result of the elimination of one iodine atom from it. Free triiodothyronine is a bioactive hormone form that is not bound to blood proteins (thyroxine-binding globulin and albumin). Free T3 makes up only ~0.4% of the total hormone. Due to the fact that it is five times stronger than T4, even such a small amount of free T3 provides all the needs of the body.
Characteristics and functions of the hormone
The main volume (80%) of triiodothyronine is created as a result of the biological transformation of thyroxine in peripheral organs (kidneys, liver), and only 20% is synthesized by the thyroid gland. T3 free is the most active form, which is involved in:
· the process of gas exchange - tissue respiration;
· heat exchange regulation;
· activation of retinol production in liver cells;
· maintaining low levels of triglycerides and cholesterol in the blood;
· increase in the rate of protein synthesis process;
· increased urinary excretion of calcium;
· destruction of architectonics and bone demineralization;
· changes in the strength and frequency of heart contractions;
· stimulation of processes occurring in some areas of the brain;
· activation of growth hormone and insulin functions;
· increased absorption of cyanocobolamin (B12) in the gastrointestinal tract;
· activation of intestinal peristalsis;
· hormone synthesizing functions of the reproductive system, etc.
Free T3 is an important diagnostic sign of thyroid function and is also used to monitor the effectiveness of its treatment.
Indications for the test
The test to determine the free T3 hormone activity is prescribed for:
· clarifying the diagnosis of thyroid pathology;
· differential diagnosis of the effectiveness of treatment of hyperthyroid conditions;
· state control in case of isolated T3-toxicosis;
· screening and differential study of the results of hormone replacement and depressant therapy using thyroxin preparations;
· abnormal level of thyroid-stimulating hormone with normal activity of T4 free;
· showing signs of hyperthyroidism and normal T4 free.
To study the free T3 level, an immunochemiluminescent analysis of venous blood is performed.
Preparation for the test
Free T3 hormone must be taken after preliminary preparation, which includes the following steps:
· 2-3 hours before blood (venous blood) drawn, exclude food intake, you can drink clean water;
· After consulting the attending physician, 2 days before visiting the laboratory, exclude a medication taking, especially hormones
· one day before the test, eliminate stress, high physical exertion on the body;
· smoking patients should refrain from smoking 3 hours prior to the analysis.
The patient is given a form in which the results of the analysis and reference values are indicated:
Age (years)
Women (pg/ml)
Men (pg/ml)
< 3
2,3 – 5,4
1,9 – 4,9
4-6
3,0 – 6,4
1,6 – 6,0
7-8
2,6 – 7,5
2,6 – 6,8
9-10
1,3 – 1,9
2,1 –4,9
11-12
2,2 – 4,7
2,3 – 5,2
13-14
2,5 – 5,2
2,6 – 5,0
15-16
2,1 – 4,2
2,3 – 5,0
17-18
2,0 – 4,4
2,1 – 4,2
19-20
2,5 – 6,2
2,0 – 4,0
21-99
1,8 – 4,2
1,8 – 4,2
The analysis is deciphered by comparing the norm and the results obtained.
Interpreting analysis results
Deviation of the T3 free hormone level from the norm indicates the presence of the following pathologies. Values are upper at:
· hyperthyroidism primary and secondary;
· isolated T3-toxicosis;
· thyroiditis;
· hypothyroidism with thyroxin resistance;
· in women - thyroid dysfunction after childbirth;
· iodine deficiency in the body;
· obesity;
· hemodialysis;
· chorioncarcinoma;
· endemic goiter, etc.;
Values are lower at:
· hypothyroidism I, II, III;
· undernutrition or low protein diet;
· stress;
· smoking;
· chronic liver pathologies;
· among women:
· high physical activit;
· I and III trimester of gestational period.
Therapy using the following medication can lead to a deviation from the norm:
· increasing the level of free T3:
· insulin;
· opiate group (methadone, heroin);
· hormones (estrogen, thyroxine);
· H2-histamine receptor blockers;
· oral contraceptives;
· phenothiazines;
· lowering the level of T3 free:
· androgens and anabolic steroids;
· salicylates;
· coumarin group;
· lithium;
· heart medications;
· β1-adenoblockers, etc.
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