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Free T3

25 Azn
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Free T3 (triiodothyronine) is a hormone secreted by the thyroid gland. It stimulates oxygen uptake by tissues and regulates metabolism. Most T3 in the body is protein-bound, while a small portion (0.2–0.5%) remains free and circulates in the bloodstream.

T3 is more biologically active than T4, although its concentration in the blood is lower. It promotes the synthesis of vitamin A in the liver, reduces cholesterol and triglyceride levels, and accelerates protein metabolism. T3 also increases calcium excretion through urine and activates bone tissue metabolism. Additionally, it has a positive chronotropic and inotropic effect on the heart.

Between the ages of 11 and 15, T3 levels reach adult concentrations. During pregnancy, T3 levels decrease from the first to the third trimester. Excessive T3 levels in newborns normalize within one week after birth. In general, T3 levels in women are 5–10% lower than in men. Seasonal variations occur, with T3 peaking between September and February and reaching its lowest levels in late spring.

  • The test is performed on a venous blood sample.
  • Blood should be drawn in the morning after 8–14 hours of fasting (water intake is allowed).
  • The test can also be performed 4 hours after a light meal.
  • For consistent monitoring, the test should always be performed at the same time of day.
  • Any interference with thyroid function may affect test results.
  • Differential diagnosis of thyroid diseases
  • Monitoring of isolated T3-toxicosis

Causes of Increased T3 Levels

  • Thyrotropinoma
  • Toxic goiter (Graves' disease)
  • Isolated T3-toxicosis
  • Thyroiditis
  • Thyrotoxic adenoma
  • Thyroid hormone resistance syndrome
  • TSH-independent thyrotoxicosis
  • Postpartum thyroid dysfunction
  • Choriocarcinoma
  • Multiple myeloma with high IgG levels
  • Nephrotic syndrome
  • Hemodialysis
  • Chronic liver diseases

Causes of Decreased T3 Levels

  • Severe (non-thyroidal) somatic and psychiatric diseases
  • Recovery phase after a severe illness
  • Primary, secondary, and tertiary hypothyroidism
  • Protein-restricted or low-calorie diets
  • Intense physical exertion in women
  • Significant weight loss
  • Medications: Amiodarone, high-dose Propranolol, iodine-containing contrast agents
  • Uncompensated primary adrenal insufficiency