Diagnostic Use
Used in investigation of thyroid function, as an adjunct to TSH, which is the preferred test for assessment of thyroid status. Also used for monitoring patients on thyroid replacement therapy.
Increased in hyperthyroidism, and in hypothyroidism being treated with thyroxine
Decreased in hypothyroidism (and in late pregnancy)
Fluctuating levels may be seen in euthyroid patients during acute or chronic illness
Temporary hypothalamic/pituitary dysfunction (aka ‘sick euthyroid syndrome’) may occur with acute illness and can create a confusing thyroid function pattern. Therefore, it is generally NOT recommended to assess thyroid function when a patient is acutely unwell, unless it is believed that thyroid dysfunction is the basis of the illness.
Test Method
Principle: Electrochemiluminescence - Competition principle
Analyser: Roche Diagnostics Cobas e801
Reagents: Elecsys FT4 IV
Limitations / Interference
Free T4 and Free T3 assays attempt to measure a very small fraction of free hormone against a much larger background of bound hormone. Consequently, these assays are prone to give incorrect estimates of free hormone when the concentration or affinity of binding proteins (TBG, transthyretin and albumin) are abnormal, or when drugs displace the hormone from the binding proteins. These effects vary with the assay used.