T3 free

The role of free T3 (triiodothyronine) in the body

T3 free (FT3) is the active form of thyroid hormone produced by the thyroid gland (20%) and formed in tissues by conversion from T4 (80%). It plays a key role in the regulation of:

Main functions:

1. Metabolism -accelerates metabolism, increases oxygen consumption and heat generation.
2. Energy balance -stimulates the breakdown of fat and glycogen.
3. Cardiovascular system -increases the heart rate (HR) and cardiac output.
4. Nervous system -supports cognitive functions, reaction speed.
5. Growth and development -critical for children (especially for the brain and bones).
6. Tissue regeneration -affects protein synthesis and cellular renewal.

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Symptoms of free T3 deficiency (hypothyroidism)

Reasons:

• Primary hypothyroidism (autoimmune thyroiditis, iodine deficiency, removal of the thyroid gland).
• Secondary hypothyroidism (pituitary/hypothalamic dysfunction – low TSH).
• Low T3 syndrome (with severe diseases, starvation, stress – the body reduces the conversion of T4→T3).

Symptoms:

• Chronic fatigue, weakness, drowsiness.
• Chilliness, cold intolerance.
• Weight gain (despite a normal appetite).
• Dry skin, brittle hair and nails.
• Slow heart rate (bradycardia), low blood pressure.
• Depression, memory loss, ‘fog in the head’.
• Constipation, swelling (especially of the face and limbs).
• Violation of the menstrual cycle in women.

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Symptoms of excess free T3 (hyperthyroidism, thyrotoxicosis)

Reasons:

• Graves ‘ disease (autoimmune hyperthyroidism).
• Toxic adenoma / goiter (hormone-active thyroid nodules).
• Thyroiditis (inflammation with a temporary release of hormonesHormones are biologically active substances that are produced by specialized cells or glands (such as endocrine glands) and regulate various physiological processes in the body. They act as chemical signals that are transmitted through the bloodstream to organs and tissues to control and coordinate a wide range of functions, including metabolism, growth and development, reproduction, mood, and more. Examples include insulin, testosterone, estrogen, and adrenaline.).
• Overdose of thyroid medications (L-thyroxine, T3).

Symptoms:

• Weight loss(despite increased appetite).
• Tachycardia, arrhythmia, sweating.
• Hand tremor, nervousness, insomnia.
• Heat intolerance, hot flashes.
• Diarrhea, frequent urination.
• Exophthalmos (bug-eye in Graves ‘ disease).
• Muscle weakness, especially in the shoulders and thighs.

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Norm of free T3 in the blood test

Age/Group	FT3 (pmol/L)	FT3 (pg/mL)*
Adults	3.1–6.8	2.0–4.4
Children	Up to age 10: higher than adult reference range	—
Pregnant women	Decreased (due to increased TBG and total T4-binding proteinsProteins are high-molecular organic substances consisting of alpha-amino acids linked in a chain by a peptide bond. In living organisms, the amino acid composition of proteins is determined by the genetic code. During synthesis, 20 standard amino acids are used in most cases. Many combinations of them determine the great diversity of properties of protein molecules. Proteins play a key role in the immune response and can perform transport, storage, catalytic, structural, and receptor functions. Proteins are an important part of the nutrition of animals and humans. The main sources of proteins are meat, poultry, fish, milk, nuts, legumes, and grains.)	—

Reference values depend on the laboratory, but on average:

* Translation: 1 pmol / l ≈ 0.65 pg / ml

Important nuances:

• T3 is more active than T4, but its blood level is lower.
• If thyroid pathology is suspected , TSH, FT4, and antibodies (AT-TPO, AT-TG) are checked.
• In the elderly, the symptoms of hypo- / hyperthyroidism may be erased.

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What should I do if there are deviations?

If T3 is lowered:

1. Check TSH and T4 (to distinguish primary hypothyroidism from secondary hypothyroidism).
2. Antibody testing(if autoimmune thyroiditis is suspected).
3. Correction of iodine deficiency (if it is caused by it).
4. L-thyroxine replacement therapy (prescribed by an endocrinologist).

If T3 is raised:

1. Ultrasound of the thyroid gland (exclude nodules, inflammation).
2. Analysis for TSH receptor antibodies (TRAb) in Graves ‘ disease.
3. Treatment: thyrostatics (thiamazole), radioactive iodine, surgery.

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Conclusion

• Free T3 is the main ‘activator’ of metabolism, affecting the heart, brain and energy.
• Deficiency leads to a slowdown in all processes (hypothyroidism).
• Excess causes hypermetabolism (thyrotoxicosis).
• The norm: 3.1-6.8 pmol / L, but the result should be interpreted together with TSH and T4.