Free thyroxine belongs to the thyroid hormones. It is also often referred to as CT4. A study of the amount of thyroxine in the blood plasma is carried out with suspicion of various endocrinological diseases, which are accompanied by symptoms of thyrotoxicosis or hypothyroidism. But first it is necessary to reveal the physiological function of free thyroxine, as well as its significance in the human body.
Physiology of thyroid hormones
The thyroid gland is an important organ for the regulation of metabolic processes in the human body. It is capable of accelerating or inhibiting the course of biochemical reactions in cells. And this is done through the secretion of hormones, that is, biologically active substances – triiodothyronine and thyroxine (T4). Once they enter the systemic circulation, they are carried to all organs and tissues. In the blood, most of the thyroxin creates compounds with blood proteins, especially with albumin and globulins. At the same time, only the free part of the hormone retains biological activity, which makes up 10% of its total amount. Therefore, it is especially important in diagnostics to study this particular fraction of thyroxine.
Normally, the hormone free thyroxine plays an important role in many physiological processes:
- growth and differentiation of tissues (especially in children);
- stimulation of protein, fat and carbohydrate metabolism;
- an increase in energy costs and the use of oxygen by cells;
- regulation of the activity of excitation processes in the central nervous system;
- decreased blood clotting ability;
- stimulation of the sympathetic autonomic system.
The regulation of the amount of free thyroxine in the blood occurs according to the principle of feedback with the pituitary gland, where thyroid stimulating hormone (TSH) is synthesized and secreted. An increase in its concentration has an inhibitory effect on the thyrocytes of the thyroid gland. As a result, the activity of cell enzymes decreases, the absorption of iodine and the secretion of thyroxine and triiodothyronine decrease.
The rate of free thyroxine in the blood
The rate of this hormone is not particularly different in different age groups. Only during pregnancy is it possible to lower the standard indicator.
| Age | Indicators of the norm (pmol / l) |
| Up to 4 months | 11.5 – 28.3 |
| 4 months to 1 year | 11.9 – 25.6 |
| 1 to 7 years old | 12.3 – 22.8 |
| 7 – 12 years old | 12.5 – 21.5 |
| 12 – 20 years old | 12.6 – 21.0 |
| Over 20 years old | 10.8 – 22.0 |
Increasing the level of free thyroxine
If the concentration of this hormone is increased, then a picture of the development of thyrotoxicosis is observed. In this case, the patient reveals the following symptoms:
- enlargement and / or induration of the thyroid gland, its asymmetry;
- cardiac tachycardia (heart rate more than 100 beats per minute);
- excessive sweating;
- nervous irritability, fussiness of the patient;
- tremors of the limbs or the whole body;
- increased body temperature (low-grade fever);
- increase in pulse blood pressure (due to systolic blood pressure);
- weight loss;
- excessive sweating;
- exophthalmos;
- thyrotoxic cardiomyopathy;
- left ventricular hypertrophy;
- accelerated breathing;
- dyspeptic symptoms (diarrhea, abdominal pain, vomiting);
- dysfunction of the sex glands;
- decreased sex drive;
- muscle pain (myopathy).
This symptomatology may indicate the development of many somatic and endocrinological diseases:
- diffuse toxic goiter;
- acute or subacute thyroiditis;
- benign neoplasms of the thyroid gland (adenoma);
- the consequences of chemotherapy;
- radiation sickness;
- exposure to certain medications (amiodarone);
- renal and hepatic impairment;
- long-term heparin therapy.
Decrease in the level of free thyroxine
A decrease in the level of free thyroxine is accompanied by the development of a clinical picture of hypothyroidism. The patient is usually lethargic, lethargic. His skin is dry, cold, and scaly (especially on the head, feet and palms). Hair loss, the appearance of edema on the face, neck, limbs is observed.
It is also characterized by memory impairment, a slow reaction to external stimuli, a decrease in concentration and performance. Sometimes patients suffer from depression, internal fears, and sleep disturbances. The patient’s body weight increases. He suffers from chronic constipation, which does not respond well to drug therapy. On the part of the cardiovascular system, there is a decrease in systolic and pulse blood pressure, the development of hypothyroid cardiomyopathy.
In the blood, you can find signs of inhibition of the proliferation of blood cells in the bone marrow – a decrease in the number of erythrocytes, platelets and leukocytes. Hemoglobin is also often low (iron deficiency anemia).
A decrease in the level of free thyroxine may indicate:
- about autoimmune diseases of the thyroid gland;
- about alimentary iodine deficiency;
- about tumors of the adenohypophysis;
- about trauma or surgical damage to the thyroid gland;
- about chronic thyroiditis;
- about infiltrative processes in the thyroid gland;
- on taking certain medications (iodine, lithium, cytokines, interferons);
- congenital thyroid defect.
Also, against this background, thyrotropin may increase, which indicates an increased function of the adenohypophysis.