Canis ISSN: 2398-2942

Thyroxine assay (free)

Synonym(s): Free thyroxine

Contributor(s): Vetstream Ltd

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 assay for analogue values.

Specificity

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

Technique (intrinsic) limitations

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 VetMedResource and PubMed.
  • Behrend E N, Kempannien R J & Young D W (1998) Effect of storage conditions on cortisol, total thyroxine and free thyroxine concentrations in serum and plasma of dogs. JAVMA 212, 1564-1568.
  • Larsson M G (1988) Determination of free thyroxine and cholesterol as a new screening test for hypothyroidism. JAAHA 24, 209-217.
  • Peterson M E, Melian C & Nicholls R (1997) Measurement of serum total thyroxine, triiodothyronine, free thyroxine and thyrotropin concentrations for diagnosis of hypothyroidism in dogs. JAVMA 211, 1396-1402.


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