Felis ISSN 2398-2950

Thyroid: T4 assay

Synonym(s): Thyroid hormone, total thyroxine, tT4, Thyroxine assay

Contributor(s): Carmel Mooney, Mark Peterson, Roger Powell


  • Thyroxine (total thyroxine/tT4/T4) is the main secretory product of thyroid gland, as both free and protein bound forms.
  • Over 99.9% of thyroxine is reversibly bound to carrier proteins.
  • The minimal free unbound fraction (free T4/fT4/fT4eq/fT4d) is metabolically active and also converted to tri-iodothyronine (T3) for cellular effects.
  • Diagnostic assays for thyroxine measure both protein-bound (inactive) and free forms.
  • Free T4 assays are designed to measure only the free unbound component but have varied accuracy.


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  • Requires validated assays.
  • Varied Fluorescent and Enzymatic ImmunoAssays (EIAs) Enzyme linked immunosorbent assay (ELISA) or RadioImmunological Assays (IRMA / RIAs).
  • Laboratory testing should ideally use external quality assurance schemes to validate performance (eg VEEEQAS / ESVE).


  • In house and most commercial laboratories (EIAs), few reference laboratories (RIAs).


  • Varies with study, test methodology and reference method used, historically often relative to dynamic testing and/or thyroid gland histopathology.


  • 95% (on average).


  • 88% (on average).

Technique (intrinsic) limitations

  • Spectrophotometric in-house analyzers are often more prone to hemolytic and lipemic interference-artifact.

Technician (extrinsic) limitations

  • Minimal if test set up and QC material performed well.

Result Data

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Further Reading


Refereed papers

  • Recent references from VetMedResource and PubMed.
  • Peterson M E et al (2015) Evaluation of Serum Thyroid‐Stimulating Hormone Concentration as a Diagnostic Test for Hyperthyroidism in Cats. JVIM 5, 1327-1334 PubMed.
  • Mooney C T, Little C J L & Macrea 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.
  • Peterson M E & Gamble D A (1990) Effect of non-thyroidal illness on serum thyroxine concentrations in cats - 494 cases. JAVMA 197, 1203-1211.
  • McLoughlin M A, DiBartola S P, Birchard S J & Day D G (1993) Influence of systemic non-thyroidal illness on serum concentration of thyroxine in hyperthyroid cats. JAAHA 29, 227-234.
  • Thoday K L & Mooney C T (1992) Historical, clinical and laboratory features of 126 hyperthyroid cats. Vet Rec 131, 257-264.

Other sources of information

  • Peterson M E (2000) Hyperthyroidism. In: Ettinger S J, Feldman E C (eds): Textbook of Veterinary Internal Medicine; diseases of the Dog and Cat. 5th edn. Philadelphia, W B Saunders Co., pp 1400-1419.
  • Feldman and Nelson's (2015) Canine and Feline Endocrinology and Reproduction. 4th edn, Elsevier Science, USA.