Serum T3 may be assayed by the same technique as T4-RIA. Total serum T3 (T3-RIA) is a specific, direct measurement of T3 using anti-T3antibody and should not be confused with the test formerly called the T3U. As previously mentioned, the T3U (THBR) is primarily an estimate of serum protein unsaturated binding sites, which secondarily provides an indirect estimate of T4 but not of T3. It is unfortunate that many used to speak of the T3U as “the T3 test,” making it difficult to be certain whether T3U or T3-RIA was meant. Analogous to T4 assay, serum T3 consists mostly of protein-bound T3. Therefore, serum T3 measurement is affected by alterations in thyroxine-binding proteins in the same direction as serum T4, although to a slightly lesser degree than serum T4. One may obtain a free T3 index using T3-RIA and THBR (T3U) values, The T3-RIA may be elevated for 1-2 hours after T3 (liothyronine) administration. It may also be temporarily increased for several hours after desiccated thyroid intake

Results in thyroid disease. Under usual circumstances, T3-RIA has at least as good sensitivity as T4 in detecting thyrotoxicosis. In fact, some investigators have stated that T3-RIA is the most sensitive test for standard hyperthyroidism associated with increase of both T4 and T3, occasionally demonstrating elevation at an early stage before T4 values have risen above reference range upper limits. In addition, T3-RIA helps to detect that form of hyperthyroidism known as “T3 toxicosis” in which the T3 level is elevated but not the T4 level. Triiodothyronine toxicosis has been estimated to comprise about 3%-4% of hyperthyroid patients.

Drawbacks. Unfortunately, T3-RIA has several substantial drawbacks. First, although T3-RIA test kits are as easy to use as T4-RIA kits, there seems to be more variation in results among T3 kits from different manufacturers than among T4 kits. Second, as noted previously, T3-RIA is affected by thyroxine-binding protein alterations similarly to T4. Third, perhaps the most serious drawback is the strong tendency of many severe acute or chronic non thyroid illnesses to decrease T3-RIA values even though the patient remains clinically euthyroid. In many of these cases T4 conversion to T3 in peripheral tissues is temporarily decreased and instead is shunted toward reverse T3 production. The decrease associated with severe non thyroid illness varies but often is very substantial and is the most common cause for artifactual T3-RIA decrease. Therefore, this severely decreases the usefulness of the T3-RIA in hospitalized patients. Fourth, T3-RIA is not reliable in hypothyroidism, because there is considerable overlap between values from hypothyroid patients and the lowormal reference range. A few reports suggest that occasionally persons have mildly hypothyroid T4 levels but enough T3 secretion by the thyroid to maintain a clinically euthyroid state. Fifth, T3-RIA is increased in iodine deficiency.

A sixth problem affecting T3-RIA is difficulty in defining the reference range. Persons over age 60 may have reference limits that are significantly lower than those for persons under age 60. Most studies have found a10%-30% decrease in mean values after age 60, although reports have varied from 0%-52%, possibly because the degree of age effect differs between individual manufacturer’s kits or there may have been some differences in the populations tested. Unfortunately, very few laboratories determine age-related values for the particular T3-RIA kit that they use. If the kit used by any individual laboratory is affected, this implies that since a result in an elderly person within the reference range but near the upper limit of the range could be artifactually decreased, that apparent normal value might in fact be elevated for that patient if the reference range was not age-corrected. A few studies report a lifelong decrease of 5-10 ng/ml/ 10 years. This implies that T3-RIA values in childhood are higher than those for adults. Finally, certain medications (e.g., propranolol, dexamethasone) have been reported to decrease T3-RIA levels, although not severely.