Understanding your Triiodothyronine Test Results

Triiodothyronine (T₃) is a hormone produced by the thyroid gland. The thyroid gland, located in the neck, produces hormones that have diverse effects on the body and its metabolism. The primary hormone produced by the thyroid is thyroxine (T₄). Both of these hormones are produced in response to the action of thyroid stimulating hormone (TSH) on the thyroid, and secreted into the blood, where T₄ is processed to generate more T₃; both hormones circulate while bound to certain proteins, including thyroid-binding globulin (TBG).

Circulating T₃ enters cells throughout the body, and binds to receptors in their nuclei; this binding regulates gene expression to exert a number of downstream effects. T₃ has diverse effects on multiple tissues throughout the body. Generally speaking, it increases body temperature, oxygen consumption, tissue and bone growth, contraction of the heart, absorption of nutrients from the intestines, formation of cholesterol (LDL) receptors, lipolysis of body fat, and overall metabolic rate. In effect, it is a metabolism-activating hormone.

Physicians may be interested in testing thyroid hormones for patients who present with symptoms typical of thyroid dysfunction, including overactive (hyper) or underactive (hypo) thyroid. Thyroid blood tests are also ordered when a thyroid goiter or an enlarged thyroid is felt on physical exam. The routine thyroid tests include T₄ and TSH levels, but T₃ levels are tested when a patient is suspected to have hyperthyroidism, but their T₄ level is normal. Although T4 is normally elevated in hyperthyroidism, in some cases T₄ is normal, while T₃ is high, still resulting in hyperthyroidism. This is called T₃ thyrotoxicosis, and it occurs when the thyroid produces excess T₃, but normal levels of T₄ (about 5% of hyperthyroid patients). Thus, for these patients, the only abnormal values may be triiodothyronine test results. In order to test hormone levels, blood will be drawn from the patient’s veins, likely from the arm or hand.

T₃ test results are reported as the amount of hormone in the blood in units of ng/dL. The normal range of T₃ is 80-200ng/dL. High T₃ levels indicate either hyperthyroidism or an increase in the TBG protein that binds to T₃ in the bloodstream. Low T₃ generally indicates hypothyridism or a decrease in TBG. Levels can also be lowered by the drug amiodarone.

Gomella LG, Haist SA, "Chapter 4. Laboratory Diagnosis: Chemistry, Immunology, Serology" (Chapter). Gomella LG, Haist SA: Clinician's Pocket Reference, 11ed.

Jameson J. L, Weetman Anthony P, "Chapter 335. Disorders of the Thyroid Gland" (Chapter). Fauci AS, Braunwald E, Kasper DL, Hauser SL, Longo DL, Jameson JL, Loscalzo J: Harrison's Principles of Internal Medicine, 17e