Clinical Information

Thyroid hormone 3,5,3’ Triiodothyronine (T3) circulates in the blood as an equilibrium mixture of free and protein-bound hormone. T3 is bound to thyroxine binding globulin (TBG), prealbumin, and albumin, and is such that only 0.2-0.4% of total T3 is present in solution as unbound or free T3. This free fraction represents the amount of physiologically active thyroid hormone.

It has become apparent in recent years that T3 plays an important role in the maintenance of the euthyroid state. Serum T3 measurements can be a valuable part of a thyroid screening panel in diagnosing certain disorders of thyroid function as well as conditions caused by iodine deficiency. Clinically, serum T3 measurements are especially valuable in diagnosing hyperthyroidism and in following the course of therapy for this disorder. Under conditions of strong thyroid stimulation, T3 measurement provides a good estimation of thyroid reserve. Recognition of a thyroid dysfunction called T3-thyrotoxicosis, associated with an increased serum T3 level but normal thyroxine (T4) and free T4 (FT4) results further highlight the importance of this assay.

Dietary iodine deficiency results in inadequate production of thyroid hormones despite presence of normal thyroid tissue. In such cases, serum T4 concentration is often low while thyroid stimulating hormone (TSH) concentrations are elevated. Elevated TSH associated with low T4 is normally indicative of hypothyroidism. However, in iodine deficiency, these results together with normal or slightly elevated serum T3 are indicative of euthyroid status in most individuals.

T3 levels are also affected by conditions which affect TBG concentration. Slightly elevated T3 levels may occur in pregnancy or during estrogen therapy, while depressed levels may occur during severe illness, malnutrition, in renal failure and during therapy with anti-thyroid drugs, propranolol and propylthiouracil and salicylates. In patients with severe or chronic illnesses, many abnormalities of thyroid hormone balance occur. T4 production and the extent of serum thyroid hormone binding may be independently abnormal, resulting in a low, normal, or high FT4 estimate. Serum T3 concentrations are often low; TSH levels may be normal or slightly elevated. Total T3 values can be helpful if hyperthyroidism is suspected and the FT4 estimate is normal.