The measurement of total triiodothyronine (T3) is a complementary test used to check for hyperthyroidism in patients with low TSH values and normal thyroxine (T4) levels and diagnose triiodothyronine toxicosis.
More information
Triiodothyronine (T3) is a hormone produced in small amounts by the thyroid gland and peripheral tissues from the conversion of thyroxine (T4). 99.96% of T3 is bound to proteins (thyroxine-binding globulin, thyroxine-binding prealbumin, and albumin), and the remainder is the biologically active form, free triiodothyronine (FT3). Approximately four times more free thyroxine (FT4) circulates than T3, partly due to its lower affinity for plasma proteins. In addition, T3 has a shorter half-life than T4.
Biologically active T3 stimulates the basal metabolic rate, including carbohydrate and lipid utilization, protein synthesis, calcium release from bone, and vitamin metabolism. In infants, T3 plays a vital role in the development and maturation of the central nervous system. Circulating T3 influences the release of thyroid-stimulating hormone (TSH) and hypothalamic thyroid-releasing hormone (TRH) through a negative feedback mechanism. T3 levels are used to confirm the diagnosis of hyperthyroidism when T4 levels are borderline high and to aid in diagnosing T3 thyrotoxicosis.
In hyperthyroidism, T4 and T3 levels are usually elevated, but in a small percentage of patients, only T3 is elevated (T3 toxicosis).
In hypothyroidism, both T4 and T3 levels are decreased. T3 levels are often low in sick or hospitalized euthyroid patients.
Possible Interpretations of Pathological Values
- Increase: Congenital increase in thyroxine-binding globulin, familial dys-albuminemic hyperthyroxinemia, fasting, Graves' disease, living at high altitudes, hyperthyroidism, pregnancy, psychiatric diseases (acute), T3 thyrotoxicosis. Medications: Amiodarone (rare), antithyroid drugs, dextrothyroxine, tromethamine, estrogens, heroin, lithium, L-triiodothyronine, methadone, oral contraceptives, rifampicin, terbutaline, thyroxine
- Decrease: Anorexia nervosa, eclampsia, elderly patients, genetic deficiency of thyroxine-binding globulin, goiter (caused by iodine deficiency), cirrhosis of the liver, iodine deficiency (severe), myxedema, obesity, hemodialysis, after surgery (caused by stress), preeclampsia, radioactive iodine therapy, renal failure, intense and prolonged fasting, thyroidectomy. Medications: Amiodarone, androgens, antithyroid drugs, asparaginase, cimetidine, dexamethasone, fenclofenac, fenoprofen, iodinated contrast agents, iopanoic acid, isotretinoin, lithium compounds, phenytoin, propranolol, propylthiouracil, salicylates, valproic acid
Important Note
Laboratory test results are the most critical parameter for diagnosing and monitoring all pathological conditions. Between 70 to 80% of diagnostic decisions are based on laboratory tests. Correctly interpreting laboratory results allows a doctor to distinguish "healthy" from "diseased."
Laboratory test results should not be interpreted from the numerical result of a single analysis. Test results should be analyzed based on each case and family history, clinical findings, and the results of other laboratory tests and information. Your physician should explain the importance of your test results.
At Diagnostiki Athinon, we answer any questions you may have about the test you perform in our laboratory and contact your doctor to ensure you receive the best possible medical care.
