Specimen Information

Type

Gold, SST

Volume

1.0 ml

Transport Info

Centrifuge and immediately transfer serum to separate plastic tube
Frozen

Fasting Required?

False

Clinical Significance

The importance of the thyroid hormones results from their influence on the general metabolic rate; both thyroxine (T4) and triiodothyronine (T3) increase the metabolic rate of most tissues. T3 is considerably more active than T4. Reverse T3 (rT3) differs from normal T3 in the positions of the iodine atoms attached to the aromatic rings. It has no effect on metabolic rate and, indeed, may be a waste product. It is probable that the ultimate regulation of metabolic activity is by the relative rates of formation of T3 and rT3 from T4. In normal, healthy individuals, levels of all three hormones tend to change together. In two groups of disease, however, this pattern is changed. In many thyroid abnormalities, T3 levels change less than do those of T4; levels of rT3 tend to follow those of T4. When T4 is low (in hypothyroidism), conversion to rT3 is reduced so T3 formation can proceed as normally as possible. Conversely, with high levels of T4, rT3 formation is increased at the expense of T3. Thus, the ratio of T3 to rT3 may be a more useful indicator of the thyroid status than either measurement alone. 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; 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.