Reverse T3 (rT3) differs from thyroid hormones in that it has no effect on metabolic rate and indeed may be a waste product. Patients with abnormalities of energy metabolism may also show significant variations in serum rT3 levels. Caloric deprivation (fasting) usually results in higher concentrations of rT3, as do various systemic diseases involving fever. This appears to result from the conversion of T4 to rT3 being favored at the expense of T3 production. Hence, rT3 measurements may be useful in the diagnosis of the “sick euthyroid” syndrome. Most drugs that affect thyroid function also change rT3 levels.

Disorders associated with increased rT3 values include:

·          Hyperthyroidism

·          Systemic febrile illnesses

·          Administration of amiodarone

·          Fasting

·          Administration of radiographic contrast agents

Disorders associated with decreased rT3 values include:

·          Hypothyroidism

·          Administration of estrogens or progestogens

·          Administration of corticosteroids

·          Administration of anti-thyroid drugs