Thyroxine, Total (TT4)
METHODOLOGY
Electrochemiluminescence Immunoassay (ECLIA)
PERFORMED
Tuesday & Friday
REPORTED
2 days
COLLECT
One 6mL RT (Min: 4mL RT)
TRANSPORT
1 mL refrigerated serum (Min: 0.5 mL)
STABILITY
Stored at 2 - 8°C until used; Stable if frozen at - 20°C
UNACCEPTABLE SPECIMENS
Do not use heat-inactivated samples.
REFERENCE RANGE
4.5 - 12.5 µg/dL
CPT CODE
84436
INFORMATION
The hormone thyroxine (T4) is the main product secreted by the thyroid gland and is an integral component of the hypothalamus-anterior pituitary-thyroid regulating system. It has the function of anabolically influencing metabolism. Thyroxine is formed in a coupling reaction from two DIT molecules (3,5-diiodotyrosine) in the thyroid gland. It is stored bound to thyroglobulin in the lumina of the thyroid follicles and is secreted as required under the influence of TSH. The major part (>99%) of total thyroxine (T4) in serum is present in protein-bound form. As the concentrations of the transport proteins in serum subject to exogenous and endogenous effects, the status of the binding proteins must also be taken into account in the assessment of the hormone concentration in serum. If this is ignored, changes in the binding proteins (e.g. due to estrogen-containing preparations, during pregnancy or in the presence of a nephritic syndrome etc.) can lead to erroneous assessments of the thyroid metabolic state.
The determination of T4 can be utilized for the following indications: the detection of hyperthyroidism, the detection of primary and secondary hypothyroidism and monitoring of TSH-suppression therapy.