Felis ISSN 2398-2950

Blood biochemistry: free thyroxine assay

Synonym(s): Free T4

Contributor(s): Mark Peterson

Overview

  • Thyroxine (T4) is main secretory product of thyroid gland.
  • Over 99.9% of thyroxine (T4) is reversibly bound to carrier proteins.
  • The unbound fraction (free T4) has greatest metabolic effect.
  • In veterinary medicine total T4 assays are generally more reliable than free T4 assays unless this is measured by Equilibrium dialysis.

Sampling

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Tests

Methodologies

  • Equilibrium dialysis:
    • Free T4 isolated from serum by passage through semipermeable membrane which allows free T4 but not plasma proteins to pass through.
  • Radioimmunoassay ('analogue').

Availability

  • Most commercial laboratories use analogue assays.
  • Can request equilibrium dialysis.

Validity

Sensitivity

  • High sensitivity for equilibrium dialysis.
  • Same/slightly lower than total T4 Thyroxine assayfor analogue values.

Specificity

  • Good specificity for equilibrium dialysis.
  • Same/slightly lower than total T4 Thyroxine assay for analogue values.

Technique (intrinsic) limitations

  • Anti-T4 antibodies affect both equilibrium dialysis and analogue tests for free T4.

Technician (extrinsic) limitations

  • Equilibrium dialysis is time consuming and involves several practical steps in which technician factors may be important.

Result Data

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Further Reading

Publications

Refereed papers

  • Recent references from VetMed Resource and PubMed.
  • Peterson M E, Melian C & Nichols C E (1998) Measurement of serum concentrations of total and free T4 in hyperthyroid cats and cats with nonthyroidal disease. J Vet Int Med 12, 211.
  • Mooney C T, Little C J L & MacRae A W (1996) Effect of illness not associated with the thyroid gland on serum total and free thyroxine concentrations in cats. JAVMA 208, 2004-2008.


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