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Causes of high and low thyroxine
Last reviewed: 06.07.2025
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In most patients with clinically expressed hyperthyroidism, the T 4 (thyroxine) content in the blood is elevated, and in hypothyroidism, it is reduced. However, in some cases, the T 4 (thyroxine) concentration in the blood does not reflect the functional state of the thyroid gland. These include conditions in which the TSH concentration changes. For example, the T 4 (thyroxine) concentration in the blood can be elevated with an increase in TSH. The latter can be due to a genetically determined increase in TSH content, as well as pregnancy, taking contraceptives containing estradiol derivatives, and estrogen therapy. At the same time, the T 4 concentration in the blood can be reduced by reducing the binding capacity of TSH. This is caused by the following pathological conditions: chronic severe liver disease, nephrotic syndrome, and a genetically determined decrease in TSH synthesis. Androgen therapy also reduces the binding capacity of TSH. In old age, 20% of people with a euthyroid state have a decrease in the concentration of TSH in the blood, which, in turn, leads to a decrease in the level of T 4.
Temporary increase in total T4 ( pseudothyroid dysfunction) is observed in almost 20% of patients admitted to psychiatric clinics. Total T4 (thyroxine) concentration may also increase in other diseases not associated with changes in thyroid function. In such patients, T4 ( thyroxine) usually normalizes within a few days without treatment. Pseudothyroid dysfunction is supported by decreased total T3 concentration and normal TSH levels.
Increased concentrations of total T4 ( and total T3 ) without signs of thyrotoxicosis are possible in a rare hereditary disease - generalized resistance to thyroid hormones. Despite the increased concentrations of T4 , cT4 , T3 and free triiodothyronine (cT3 ), patients have a euthyroid state, and some even have mild hypothyroidism.
Diseases and conditions in which the concentration of T 4 (thyroxine) in the blood serum changes
Increased concentration
- Hyperthyroidism
- Acute thyroiditis
- Pregnancy
- Obesity
- Hepatitis
- Use of estrogens (oral contraceptives), heroin, thyroid drugs
- Hypothyroidism (myxedema)
Decreased concentration
- Increased protein loss (renal syndrome)
- Itsenko-Cushing syndrome
- Severe iodine deficiency
- Physical activity
- Panhypopituitarism
- Protein loss through the gastrointestinal tract
- Use of glucocorticosteroids, reserpine, sulfonamides, penicillin, potassium iodide, androgens