Syndrome of low T3

Syndrome low T3 (Euthyroid Sick Syndrom) is characterized by a low content of thyroid hormones in the serum of clinically euthyroid patients with systemic diseases of non-thyroid etiology. Diagnosis is made after the exclusion of hypothyroidism. Therapy includes treatment of concomitant disease, thyroid hormone replacement therapy is not indicated.

Causes of the syndrome of low T3

Patients with different acute and chronic non-thyroid pathologies may have altered laboratory parameters characterizing the thyroid function. This pathology includes acute and chronic diseases such as malnutrition, starvation, protein and caloric malnutrition, severe trauma, myocardial infarction, chronic renal failure, diabetic ketoacidosis, anorexia nervosa, liver cirrhosis, burn wounds and sepsis.

Most often, euthyroid syndrome of low T3 is characterized by a decreased level of T3. In patients with more severe manifestations of the underlying disease or with long-term chronic diseases, a decrease in the T3 level is also observed. Whey reversible T (pT3) increased. Patients are clinically euthyroid and do not have an increase in TSH.

[1], [2], [3], [4], [5]

Pathogenesis

The pathogenesis of the syndrome is not known to date, but it is believed that it may include a reduced peripheral conversion of T to T3, a reduced clearance of pT3 produced from T3, and a reduced ability of thyroid hormones to bind to thyroxin-binding globulin (TBG). Pro-inflammatory cytokines (tumor necrosis factor a, IL-1) may be responsible for some changes.

Interpretation of changes in laboratory parameters of thyroid function is complicated due to the influence of various drugs, including contrast preparations of iodine, amiodarone, which aggravate the violation of peripheral conversion of T to T3, and due to the influence of other drugs such as  dopamine  and glucocorticoids that lower the pituitary secretion of TSH, resulting in a low serum TSH level and a subsequent decrease in T3 secretion.

[6], [7], [8], [9]

Diagnostics of the syndrome of low T3

There is a diagnostic dilemma: whether the patient has hypothyroidism or low T3 syndrome. The best laboratory test for resolution of the dilemma is the TSH level, which in the case of low T syndrome is low, normal or moderately elevated, but not as high as it could be with hypothyroidism. The serum pT level is increased, although this test is rarely performed in clinical practice. Serum cortisol is often elevated with low T3 syndrome and decreased (or normally low) with secondary and tertiary hypothyroidism (pituitary-hypothalamic pathology).

Since laboratory tests are not specific, a clinical evaluation is needed to interpret the changes in laboratory parameters of the thyroid function. Until there is a firm belief in the presence of thyroid pathology, patients in intensive care units should not be assigned functional laboratory tests of the thyroid gland.

[10], [11], [12], [13]

Treatment of the syndrome of low T3

Treatment with hormone replacement therapy is not applied; laboratory indicators are normalized with successful treatment of the underlying disease.