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Endocrinologist

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Causes of high and low free thyroxine

Alexey Kryvenko, medical expert
Last reviewed: 04.07.2025

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Diseases and conditions in which the concentration of cT4 ₍ thyroxine) in the blood serum changes

Increased concentration

Hyperthyroidism
Acute thyroiditis
Pregnancy
Obesity
Hepatitis
Use of estrogens (oral contraceptives), thyroid drugs, heparin, imidazole derivatives

Decreased concentration

Hypothyroidism (myxedema)
Increased protein loss (renal syndrome)
Itsenko-Cushing syndrome
Androgen intake
Severe iodine deficiency
Physical activity
Panhypopituitarism
Protein loss through the gastrointestinal tract
Use of glucocorticosteroids, reserpine, sulfonamides, penicillin, potassium iodide
Thyroid resection
Thyroid cancer
Antithyroid drug overdose