The measurement of free triiodothyronine (FT3) in serum is used as a complementary test for the control of hyperthyroidism in conjunction with other tests of thyroid function, for the evaluation of clinically euthyroid patients who have altered proteinuria distribution and to monitor thyroid hormone replacement therapy.
Triiodothyronine (T3) is a hormone produced in small quantities by the thyroid gland and peripheral tissues by the conversion of 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, in part 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, including carbohydrate and lipid utilization, protein synthesis, calcium release from bone, and vitamin metabolism. In infants, T3 plays an important 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 the diagnosis of 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.
In general, measurement of free T3 (FT3) is not necessary and usually measurement of total T3 is sufficient to assess thyroid function. However, measurement of FT3 may be necessary for the evaluation of clinically relevant patients with disorders in the distribution of binding proteins (such as in 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, phenoprofen, iodized silicones, iopanoic acid, isotretinoin, propylthiol, lithium compounds, phenylpropylene
Laboratory test results are the most important parameter for the diagnosis and monitoring of all pathological conditions. 70%-80% of diagnostic decisions are based on laboratory tests. Correct interpretation of 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 interpreted in relation to each individual case and family history, clinical findings and the results of other laboratory tests and information. Your personal physician should explain the importance of your test results.
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