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Free T4
Principle
Thyroxine (T4) is the principal hormone secreted by the thyroid gland and serves as a precursor to the more biologically active triiodothyronine (T3). The free fraction of circulating thyroxine (FT4) represents the hormonally active component available to tissues and is considered the most direct indicator of an individual’s thyroid functional status.
FT4 concentrations are typically decreased in hypothyroidism and increased in hyperthyroidism. Measurement of FT4 therefore provides valuable information for the differential diagnosis of thyroid disease. Because FT4 concentrations are independent of thyroid hormone–binding protein levels, accurate assessment is possible in patients with increased or decreased binding protein concentrations without the need for additional tests of binding capacity.
Specimen Requirements
Specimen
Collection
Processing
Storage and Transport
Store/transport at 15–30°C for ≤8 hours; 2–8°C for ≤48 hours; freeze at ≤ –20°C if delayed. Avoid repeated freeze–thaw cycles.
Unacceptable Specimens
Hemolyzed or unseparated specimens.
Method
Enhanced Chemiluminescence.
Normal Range
0.61–1.12 ng/dl
Turnaround Time
References
- Robbins J, Rall JE. Interaction of thyroid hormones and proteins in biological fluids. Recent Prog Horm Res. 1957;13:161–202.
- Robbins J, Rall JE. The iodine-containing hormones. In: Grady CH, Bacharach AL, eds. Hormones in Blood. Vol 1. London: Academic Press; 1967:383–490.
- Avruskin TW, et al. Measurements of free and total serum T3 and T4 in pregnant subjects and neonates. Am J Med Sci. 1976;271:309–315.
- Lewis M. Measurement of free thyroid hormones in health and disease. In: Ekins R, et al., eds. International Symposium on Free Thyroid Hormones. Venice: Excerpta Medica; 1979:167–180.