Free Thyroxine ELISA Kit (Free T4)


Human free thyroxine ELISA kit is a reliable quantitative procedure to measure free T4 (free thyroxine, FT4) concentrations in human serum. The sensitivity detection limit of this assay is 0.22 ng/dl.


L-Thyroxine (T4) has a primary influence on oxygen consumption and protein synthesis in virtually every tissue, it is also the most commonly analysed thyroid hormone during the diagnostics of thyroid function. Other functions include; growth, development and sexual maturation. Most of the serum-T4 is bound to transporter proteins (thyroxine binding globulin, thyroxine binding pre-albumin and albumin). Only a small portion approx. 0.03% exists as the active unbound (free T4) form which is responsible for exerting its effect on cells.

During the earlier thyroid function test procedures, the variable effect of thyroxine binding globulin (TBG) was overcome by calculating the free thyroxine index (FTI). The FTI is the product of thyroid uptake (TU) and total T4 concentration and it was used to assess the number of TBG binding sites that were available. This was quite time consuming protocol and required two assays to be performed (TU and total T4), but it did provide a better thyroid status level compared to total T4 alone.


All reagents supplied need to be stored at 2 °C – 8 °C, unopened reagents will retain reactivity until expiration date. Do not use reagents beyond this date.

  • Microtiterwells, 12 x 8 (break apart) strips: Wells coated with anti-T4 antigen.
  • Standard 0-5: Concentration 0,0, 0.5, 1.0, 2.0, 4.0, 8.0 ng/dl.
  • Control Low and Control High.
  • Enzyme Conjugate:  Consisting of anti thyroxine conjugated to horseradish peroxidase.
  • Substrate Solution: Tetramethylbenzidine (TMB).
  • Stop Solution: 0.5 M H2SO4, Avoid contact with the stop solution. It may cause skin irritations and burns.
  • Wash Solution (40x Concentrated).


The minimum sensitivity detection limit of human free thyroxine (free T4, FT4) using current free T4 ELISA kit is 0.22 ng/dl. The dynamic assay range for this kit is 0.5-8.0 ng/dl.


– Assay Dynamic Range: 0.22 – 8.0 ng/dl
– Specificity (Cross Reactivity): T4 (0.03%), T3 (0.01%)
– Sensitivity: LoB (0.20 ng/dl), LoD (0.271 ng/dl), LoQ (0.298 ng/dl)
– Intra-Assay Variation: 1.2 – 6.9%
– Inter-Assay Variation: 1.0 – 6.9%
– Inter-Lot Variation: 3.6 – 14.5%
– Recovery: 85.0 – 114.8%
– Linearity: 96.5 – 113.0%
– Interfering Substances: None
– High-Dose-Hook Effect: No hook effected observed.


  1. J. Biological Chemistry (1948) 173, 175. Barker, S.B.
  2. J. Clinical Endocrinol. (1971) 33, 865. Chopra, I.J., Solomon, D.H. and Ho, R.S.
  3. Clinical Chemistry (1975) 21, 3660. Young, D.S., Pestaner, L.C. and Gilberman, U.
  4. Diagnosis and Treatment of Thyroid Disease, Cleveland CRC Press (1975) p. 19-51. Sterling, L.


  • Full Name: Free Thyroxine ELISA Kit (Free T4)
  • Reactivity: Human
  • Sample Type: Serum
  • Sensitivity:  0.22 ng/dl


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