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T4 (Thyroxine)

25 Azn
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Thyroxine (T4) is synthesized by the follicular cells of the thyroid gland under the regulation of thyroid-stimulating hormone (TSH). Most T4 circulating in the blood is bound to proteins, with only the free form being biologically active. Free T4 constitutes 3–5% of the total T4 in the body.

The concentration of T4 in the blood is higher than that of T3. It accelerates metabolism, increases thermogenesis, and enhances oxygen utilization by the body (except in the brain, spleen, and gonads). This increases the body's need for vitamins and stimulates the synthesis of vitamin A in the liver. T4 reduces cholesterol and triglyceride levels in the blood and accelerates protein metabolism. It also has a positive chronotropic and inotropic effect on the heart.

  • 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.
  • Abnormal TSH levels
  • Follow-up examination for diffuse toxic goiter
  • Clinical signs of hypothyroidism or thyrotoxicosis

Causes of Elevated T4 Levels

  • Toxic goiter
  • Thyroiditis
  • Thyrotoxic adenoma
  • Thyroid hormone resistance syndrome
  • Hypothyroidism
  • Choriocarcinoma
  • Nephrotic syndrome
  • Obesity
  • Conditions associated with increased free fatty acids during heparin therapy
  • Multiple myeloma with high IgG levels
  • Chronic liver diseases
  • Porphyria

Causes of Decreased T4 Levels

Primary Hypothyroidism:

  • Congenital hypothyroidism
  • Endemic goiter
  • Autoimmune thyroiditis
  • Tumors of the thyroid gland

Secondary Hypothyroidism:

  • Sheehan’s syndrome
  • Inflammatory diseases of the pituitary gland

Tertiary Hypothyroidism:

  • Brain tumors
  • Inflammatory processes in the hypothalamus

Medication-Induced T4 Reduction:

  • Triiodothyronine
  • Methimazole
  • Propylthiouracil
  • Glucocorticoids
  • Anti-tuberculosis drugs
  • Antifungal drugs
  • Lipid-lowering drugs
  • Selective serotonin reuptake inhibitors (SSRIs)
  • Androgens
  • Anticonvulsants
  • Furosemide