The serum's free triiodothyronine (FT3) measurement is a complementary test for testing hyperthyroidism. In conjunction with other tests of thyroid function, it is used to evaluate clinically euthyroid patients with altered proteinuria distribution and monitor thyroid hormone replacement therapy.
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Triiodothyronine (T3) is a hormone produced in small quantities by the thyroid gland and peripheral tissues by converting thyroxine (T4). 99.96% of T3 binds to proteins (thyroxine-binding globulin, thyroxine-pralbumin, and albumin binding), and the remainder is the biologically active form, free triiodothyronine (FT3). About four times as much as T3, free thyroxine (FT4) is released partly because of its lower affinity for plasma proteins. In addition, T3 has a shorter half-life than T4.
Biologically active T3 stimulates basic metabolic rate (BMR), 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 affects the release of thyroid hormone (TSH) and hypothalamic thyroid hormone-releasing hormone (TRH) via a negative feedback mechanism. T3 levels are used to confirm the diagnosis of hyperthyroidism when T4 levels are marginally high and to assist in diagnosing T3 thyroid toxicity.
In hyperthyroidism, both T4 and T3 levels are usually elevated, but in a small proportion of patients with hyperthyroidism, only T3 is elevated (T3 toxicity).
Hypothyroidism reduces both T4 and T3 levels. T3 levels are often low in sick or hospitalized euthyroid patients.
Measuring free T3 (FT3) is generally unnecessary, and total T3 is usually sufficient to assess thyroid function. However, measuring FT3 may be necessary to evaluate clinically relevant patients with disorders in the distribution of binding proteins (such as pregnancy or dysalbuminemia).
Possible Interpretations of Pathological Values
- Increase: Congenital elevation of thyroxine-binding globulin, familial dysalbuminemia, hyperthyroxinaemia, fasting, Graves' disease, high altitude living, hyperthyroidism, pregnancy, psychiatric illness (acids), acidosis. Medications: Amiodarone (rare), antithyroid drugs, dextrothyroxine, tromethamine, estrogen, heroin, lithium, L-triiodothyronine, methadone, oral contraceptives, rifampicin, terbutaline, thyroxine
- Decrease: Nervous anorexia, eclampsia, elderly patients, thyroxine-binding globulin deficiency, bronchocele (caused by iodine deficiency), cirrhosis, iodide deficiency (severe), myocardial infarction (severe), stress, preeclampsia, radioactive iodine therapy, renal failure, severe 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.
