Free T4 and general T4, what is the difference

This analysis allows you to determine the content of the hormone in the blood and in case of deviations from the norm to establish an accurate diagnosis.

Thyroxine plays an important role in the body, its main function is plastic and energy metabolism. An analysis on T4 can be prescribed by any doctor – endocrinologist, gynecologist, therapist, neurologist, surgeon, oncologist, etc.

Why you need to determine the level of T4

It is very important to take the analysis not only to freedom. thyroxine, but also on the general one, since an increase in total T4 indicates abnormalities throughout the body, and the determination of the free part indicates a specific thyroid pathology.

Physically, a person feels only changes in the free part of thyroxine. High levels of the hormone in the blood are accompanied by severe irritability, palpitations, excessive sweating, tingling sensations in the heart, tremors, sudden weight loss and the inability to gain weight.

A low thyroxine content leads to weight gain, as the metabolism slows down and the degradation of BJU worsens. A person becomes lethargic and lethargic, constantly feels tired, the body begins to swell. Memory and attention are greatly impaired. This condition is especially dangerous for pregnant women – a lack of hormones can lead to congenital thyroid diseases in a child.

Very often, hormonal failure leads to violations of the menstrual cycle in women, therefore, a gynecologist with this problem must prescribe the patient an examination for hormones of the endocrine system, ovaries and adrenal glands.

Thyroxine (T4) is a hormone synthesized by the thyroid gland and responsible for the normal functioning of the organ. Thyroxine tests indicate free T4 and total T4.


Thyroxine is synthesized by the cells of the main organ of the endocrine system – the thyroid gland under the control of thyroid-stimulating hormone (TSH) and has the property to accumulate in the tissues of the organ. T4 has a longer effect on the body, so the constant maintenance of its normal level in the blood is vital.

T4 functions:

  1. It controls the metabolism.
  2. Increases heat generation in tissues.
  3. Normalizes heart rate.
  4. Accelerates the breakdown of fats and increases protein synthesis.
  5. Increases the absorption of glucose in the small intestine.
  6. Controls the synthesis of growth hormone in children.

Free T4

Free thyroxine is a part of the general hormone that plays an important biological role. Once in the blood, some of the thyroxine ( not more than 0.4% ) does not bind to blood plasma. This part is called “free” – it either turns into the hormone T3 (triiodothyrotine), or joins a specific cell and exerts its effect.

Properties of free T4:

  • It does not bind to blood proteins (plasma).
  • It moves through the cells of the body on its own with blood.
  • It is in the body for a short time.
  • Destroyed when passing through the liver and kidneys.
  • It has low concentrations.
  • Affects the hormonal background of the body.

The standard content T4 freedom. in blood (in pmol / liter):

Age Values
Infants under 4 months of age From 11.5 to 28.3
Infants 4 to 12 months From 11.9 to 25.6
Children from 1 year to 7 years From 12.3 to 22.8
Children from 7 to 12 years old From 12.5 to 21.5
Teens 12 to 20 years old From 12.6 to 21
Adults over 20 years old From 10.8 to 22

In pregnant women:

Pregnancy (in weeks) Values
0 to 13 From 12.1 to 19.6
13 to 28 From 9.6 to 17
28 to 42 From 8.4 to 15.6
Associated T4

Bound T4 is a part of the hormone that is only coupled to specific proteins. Bound thyroxine has practically no effect on the body, as it is retained by proteins, and in this state it circulates through the body for many months.

Properties of the associated T4:

  • Binds to proteins.
  • It moves to the internal organs.
  • It is in the body for a long time.
  • When passing through the liver and kidneys, it remains unchanged.
  • It has large concentrations.
  • Does not affect the hormonal background of the body.
General T4

Total T4 is the total amount of all thyroxine (free and bound) that is in the blood. The indicators allow you to determine how much hormone was released from the thyroid gland.

Normal values

The value of the total T4 content in the blood depends on age, gender, weight and other signs. Values ​​are considered normal (in nmol / liter):

Age Values
Infants under 4 months of age From 69.6 to 220
Infants 4-12 months From 73.0 to 206
Children from 1 to 7 years old From 76.6 to 189
Children from 7 to 12 years old From 77 to 178
Teens from 12 to 20 years old From 76 to 170
Adults over 20 years old From 66 to 180
Reasons for increase and decrease
Reasons for the increase Reasons for the decline
· Diffuse toxic goiter.

· Benign tumors of the organs of the endocrine system.

Inflammatory diseases of the endocrine system.

· TSH-independent thyrotoxicosis.

· Heparin therapy.

· Improper diet and overweight.

· Disorders of the endocrine system in the postpartum period.

· Diseases of the kidneys and liver.

Hypothyroidism primary and secondary.

Autoimmune thyroiditis.

An enlargement of the thyroid gland.

· Removal of the thyroid gland (including partial).


· Lack of iodine.

· Taking hormonal contraceptives.

· Heavy metal poisoning (especially lead).

· Addiction (especially heroin addiction).

· Depletion of the body.

How to take tests

In order for the test results to be as accurate as possible, the following rules must be observed:

  1. Donate blood for analysis on an empty stomach.
  2. Exclude the use of hormonal (thyroid and steroid) drugs 48 hours before the examination (after consulting with your doctor).
  3. 24 hours before the examination, exclude physical, nervous and emotional stress.
  4. Do not smoke 3-4 hours before the test.

Venous blood is given for research.

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