Triiodothyronine (T3)

The Triiodothyronine (T3), Total test (also known as total triiodothyronine or TT3) is used to assess thyroid function when thyrotoxicosis is suspected, with special utility in T3 thyrotoxicosis where T3 is elevated and T4 remains normal. It supports the diagnostic evaluation of hyperthyroidism across causes, including Graves disease and toxic nodules, and is informative when TSH is suppressed but the standard thyroid profile is normal or borderline. The assay may be used to monitor patients receiving T3 (liothyronine) therapy. In selected presentations—unexplained weight loss, tachyarrhythmias, or proximal myopathy with a non‑elevated T4—total T3 can provide additional diagnostic context.

T3 is the more potent thyroid hormone at the tissue level and is generated predominantly by peripheral deiodination of T4. In circulation, the majority of T3 is bound to thyroxine‑binding globulin, transthyretin, and albumin, with less than 1% present as the free, biologically active fraction. Because total T3 reflects both bound and free hormone, changes in binding proteins influence measured concentrations. Total T3 increases during pregnancy and with oral contraceptive use, and other binding‑protein abnormalities can also shift results. T3 concentrations may be reduced in nonthyroidal illness and during fasting, and serum T3 may remain normal in thyroxine‑mediated thyrotoxicosis. Interpretation requires correlation with the clinical setting and awareness of conditions and therapies that alter binding protein levels.