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Free T3
Principle
Triiodothyronine (T3) is a biologically active thyroid hormone that plays a central role in the regulation of metabolic activity. The free fraction of circulating T3 (FT3) represents the hormonally active component available to tissues and is considered the primary mediator of T3’s metabolic effects.
In hyperthyroidism, FT3 concentrations are typically elevated and provide effective discrimination at the euthyroid-to-toxic threshold, making FT3 a useful marker for confirming hyperthyroidism and monitoring therapeutic response. In hypothyroidism, FT3 concentrations are often reduced; however, the decrease is generally insufficient to provide definitive diagnostic information when used alone.
FT3 concentrations are independent of thyroid hormone–binding protein levels, allowing accurate assessment in patients with increased or decreased binding protein concentrations without the need for additional binding capacity tests. Measurement of FT3 should be used as part of a comprehensive thyroid function evaluation, which may include free thyroxine (FT4) and high-sensitivity thyroid-stimulating hormone (TSH) assays.
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
2.5–3.9 pg/ml
Turnaround Time
References
- Robbins J, Rall JE. Interactions of thyroid hormones and proteins in biological fluids. Recent Prog Horm Res. 1957;13:161–202.
- 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.
- Pinchera A, et al. Free thyroid hormones in thyroid disease and effect of thyroid replacement therapy: observations by frequent sampling. In: Ekins R, et al., eds. International Symposium on Free Thyroid Hormones. Venice: Excerpta Medica; 1979:208–220.
- Avalos E, et al. Diagnostic value of free triiodothyronine in serum. J Nucl Med. 1986;27:1702–1705.