Serum Separator Tube (SST)
Free thyroxine comprises a small fraction of total thyroxine. The free T4 (FT4) is available to the tissues and is, therefore, the metabolically active fraction. Free thyroxine (FT4) are most commonly used in preference to total hormone measurements (TT4 or TT3) to improve the diagnostic accuracy for detecting hypo- and hyperthyroidism in patient populations with the thyroid hormone binding abnormalities.
The exception to this general rule occurs in those select conditions where TSH may be diagnostically unreliable in which case FT4 becomes the first-line test of choice. Such conditions include periods of unstable thyroid status (the early phase of treating hypo- or hyperthyroidism), when hypothalamic-pituitary dysfunction is suspected to be present, or when patients are taking drugs such as glucocorticoids that are known to affect TSH secretion.
Current FT4 testing methods have not yet received adequate validation in the hospital setting in which a wide variety of non-thyroidal illnesses and drug therapies are frequently encountered that are known to impair the diagnostic accuracy of both thyroid hormone and TSH testing.
Discordances between TSH and FT4 measured on the same specimen can on occasion cause a diagnostic dilemma. However, it should be recognized that the intrinsic log/linear TSH/FT4 relationship dictates that modest reductions in TSH (0.05 – 0.3 mIU/L), or modest elevations (3 – 10 mIU/L), would not be expected to be associated with changes in FT4 values outside the normal reference range.
Pregnancy: Non-pregnant FT4 reference ranges do not apply to pregnancy. Specifically low FT4 immunoassay values are observed in a significant proportion of women by the third trimester of pregnancy. Albumin levels tend to fall during pregnancy and the frequency of low FT4 values during pregnancy appears to be method-related, likely reflecting the albumin-dependence of the method.
Discrepancies in free T4 levels between methods are recognized.
Free T4 results should be interpreted in light of the total clinical presentation of the patient, including: symptoms, clinical history, data from additional tests and other appropriate information.
Elevated values suggest hyperthyroidism or exogenous thyroxine. Decreased values suggest hypothyroidism. Non-thyrometabolic disorders may cause abnormal free T4 levels. Anticonvulsant drug therapy (particularly phenytoin) may result in decreased free T4 levels due to an increased hepatic metabolism. Lithium and iodide preparations lower FT4 levels. FT4 may be increased with radiologic contrast agents, propranolol, amiodarone.
Patients on heparin therapy may have elevated free T4 levels due to release of non-esterified fatty acids, this can alter the relationship between free and bound hormones.