Triiodothyronine, reverse

Reverse Triiodothyronine
Synonyms	rT3
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Reverse triiodothyronine (rT3) is an isomer of the thyroid hormone triiodothyronine (T3). It is a biologically inactive form of T3 that is produced in the body through the peripheral conversion of thyroxine (T4). Unlike T3, reverse T3 does not stimulate thyroid hormone receptors and is considered to be an inactive metabolite.

Biochemistry

Reverse triiodothyronine is formed by the enzymatic removal of an iodine atom from the outer ring of thyroxine (T4). This process is catalyzed by the enzyme iodothyronine deiodinase, specifically type 3 deiodinase (D3). The chemical structure of rT3 is similar to that of T3, but the position of the iodine atoms differs, rendering it inactive.

Physiological Role

The physiological role of reverse T3 is not fully understood, but it is believed to be involved in the regulation of thyroid hormone activity. During periods of illness, fasting, or stress, the body increases the conversion of T4 to rT3, which reduces the levels of active T3. This is thought to be a protective mechanism to conserve energy.

Clinical Significance

Reverse T3 levels can be measured in the blood, and alterations in its levels can be indicative of certain medical conditions. Elevated rT3 levels are often seen in euthyroid sick syndrome, a condition where patients have abnormal thyroid function tests but are clinically euthyroid. This syndrome is commonly observed in critically ill patients.

Euthyroid Sick Syndrome

In euthyroid sick syndrome, the conversion of T4 to T3 is impaired, leading to increased production of rT3. This results in low serum T3 levels despite normal or elevated T4 levels. The exact mechanism is not fully understood, but it is thought to be related to changes in deiodinase activity during illness.

Hypothyroidism

In some cases of hypothyroidism, patients may have elevated rT3 levels. This can occur when there is an imbalance in the conversion of T4 to T3 and rT3, leading to symptoms of hypothyroidism despite normal T4 levels.

Diagnosis

The measurement of reverse T3 is not routinely performed in clinical practice, but it can be useful in certain situations to assess thyroid function. It is typically measured using a blood test, and results are interpreted in conjunction with other thyroid function tests such as TSH, T4, and T3.

Treatment

There is no specific treatment for elevated reverse T3 levels. Management focuses on treating the underlying condition that is causing the alteration in thyroid hormone metabolism. In cases of euthyroid sick syndrome, treatment of the underlying illness often leads to normalization of thyroid function tests.

See Also

• Triiodothyronine
• Thyroxine
• Thyroid hormone
• Euthyroid sick syndrome

External Links

• [Link to relevant medical resources]