Why t3 is more potent than t4

Only the free forms are metabolically active. Total T3 consists of both the bound and unbound fractions. In hyperthyroidism, both T4 and T3 levels are usually elevated, but in a small subset of hyperthyroid patients, only T3 is elevated T3 toxicosis. In hypothyroidism T4 and T3 levels are decreased. T3 levels are frequently low in sick or hospitalized euthyroid patients.

For SI unit Reference Values, see www. Abnormal levels high or low of thyroid hormone-binding proteins primarily albumin and thyroid-binding globulin may cause abnormal T3 concentrations in euthyroid patients. Phenytoin, phenylbutazone, and salicylates cause release of T3 from the binding proteins, thus leading to a reduction in the total T3 hormone level at normal free T3 levels.

Binding protein anomalies may cause values that deviate from the expected results. Pathological concentrations of binding proteins can lead to results outside the reference range, although the patient may be in a euthyroid state. Free T3 or free T4 testing is indicated in these cases. Some patients who have been exposed to animal antigens, either in the environment or as part of treatment or imaging procedures, may have circulating anti-animal antibodies present. The number and position of the iodines is important.

Several other iodinated molecules are generated that have little or no biological activity; so called "reverse T3" 3,3',5'-T3 is such an example. A large majority of the thyroid hormone secreted from the thyroid gland is T4, but T3 is the considerably more active hormone.

Although some T3 is also secreted, the bulk of the T3 is derived by deiodination of T4 in peripheral tissues, especially liver and kidney. Deiodination of T4 also yields reverse T3, a molecule with no known metabolic activity.

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