EBM Consult

Lab Test: Triiodothyronine, Free T3 Level

    Lab Test
    • Triiodothyronine (Free T3)
    • Measurement of total triiodothyronine (T3) levels for the evaluation and management of thyroid dysfunction. 
    • Used to monitor thyroid replacement and suppressive therapy.
    Reference Range
    • Adults
      • 2.6 - 4.8 pg/mL
      • 4 - 7.4 pmol/L
    Indications & Uses
    • Suspected hyperthyroidism in patients with thyroid-stimulating hormone (TSH) levels less than 0.1 milliInternational Units/L - elevated total serum T3 levels are used to help confirm the diagnosis of hyperthyroidism in patients with low TSH levels.  T3 levels are usually increased more than thyroxine (T4) levels.
    • Suspected T3 thyrotoxicosis - T3 thyrotoxicosis is diagnosed when the T3 level is elevated and the T4 level is normal.  This state is considered a subset of hyperthyroidism.
    Clinical Application
    • Thyroid hormones are produced when tyrosine incorporates organic iodine to form a monoiodotyrosine.  This complex picks up another iodine and become diiodotyrosine. 
    • Two diiodotyrosines combine to form tetraiodothyronine (also called T4 or thyroxine). 
    • If a diiodotyrosine combines with a monoiodotyrosine, triiodothyronine (also called T3 thyroid hormone) is formed. About 80% of circulating T3 comes from the peripheral conversion of T4.  The metabolic activity of T3 is 5 to 10 times that of T4.  Only minute quantities are unbound or "free".  It is the free T3 that is metabolically active.  Total T3 levels generally correspond with free T3 levels, except when thyroid hormone-binding protein concentrations are abnormal.  To evaluate for those binding proteins, T3 resin uptake or thyroid-binding globulin should be done at the same time as the total T3.  T3 is less useful in the diagnosis of hypothyroidism because other non-thyroid diseases can decrease T3 levels by decreasing the conversion of T4 to T3 in the liver.]
    • Increased levels may indicate:
      • Primary hyperthyroid states (e.g., Graves disease, Plummer disease, toxic thyroid adenoma), acute thyroiditis, factitious hyperthyroidism, Struma ovarii, or TBG increase (e.g., as occurs in pregnancy, hepatitis, congenital hyperproteinemia)
    • Decreased levels may indicate:
      • Hypothyroid states (e.g., cretinism, surgical ablation, myxedema), pituitary insufficiency hypothalamic failure, protein malnutrition and other protein-depleted states (e.g., nephrotic syndrome), iodine insufficiency, non-thyroid illnesses (e.g., renal failure, Cushing disease, cirrhosis, surgery, advanced cancer), or hepatic disease
    • Results decreased in severe systemic illness.
    Related Tests
    • Transplant panel
    • Long-acting thyroid stimulator (LATS)
      • Used to support the diagnosis of Graves disease, especially when the diagnosis is complex.
    • Thyrotropin-releasing hormone (TRH) stimulation test
      • Helpful in the differential diagnosis of hypothyroidism
    • Thyroxine-binding globulin (TBG)
      • Used in the evaluation of patients who have abnormal total T4 and T3 levels. 
    • Thyroxine (T4), free
      • The FT4 is used to evaluate thyroid function in patients who may have protein abnormalities that could affect total T4 levels. 
    • Antithyroglobulin antibody
      • Primarily used for the differential diagnosis of thyroid diseases, such as Hashimoto thyroiditis and chronic lymphocytic thyroiditis (in children).
    Drug-Lab Interactions
    • Radioisotope administration before the test may alter the results, if this test is performed by RIA methods. 
    • Pregnancy, if measuring Total T3. 
    • Drugs that may cause increased levels include:  estrogen, methadone, and oral contraceptives.
    • Drugs that ay cause decreased levels include:  anabolic steroids, androgens, phenytoin (Dilantin), propranolol (Inderal), reserpine, and salicylates (high dose).
    Test Tube Needed
    • Red top tube
    • Determine whether the patient is taking any exogenous T3 medication, because this will affect test results. 
    • Withhold drugs that may affect results (with physician's approval).
    • Collect a venous blood sample.
    • Apply pressure or a pressure dressing to the venipuncture site and observe the site for bleeding.
    Storage and Handling
    • Store at room temperature or preferably at 4°C for up to 7 days.
    • Store at -20°C for up to 30 days.
    What To Tell Patient Before & After
    • Explain the procedure to the patient. 
    • Tell the patient that no fasting is required.
    • LaGow B et al., eds. PDR Lab Advisor. A Comprehensive Point-of-Care Guide for Over 600 Lab Tests.  First ed. Montvale, NJ: Thomson PDR; 2007.
    • Pagana K, Pagana TJ eds. Mosby's Manual of Diagnostic and Laboratory Tests. 5th Ed.  St. Louis, Missouri. 2014.

MESH Terms & Keywords

  • Triiodothyronine, Free T3,T3 Lab Test, Free T3 Lab, Thyroid Labs