Equis ISSN 2398-2977

Endocrine: T3 assay

Synonym(s): Total T3 Assay, tri-iodothyronine assay

Contributor(s): Kathleen P Freeman, Janice Sojka, Han van der Kolk


  • Often available as part of laboratory profile, measured in conjunction with total T4.
  • 10-20% of circulating T3 secreted from thyroid gland with remainder derived from peripheral T4 degradation.
  • T3:T4 ratio of approximately 23:1 in horses compared to 20:1 usually seen in other species.
  • No utility in evaluation of T3 without concurrent T4 assay.
  • In the horse also reverse T3 (rT3) exists.
  • Free T4 0.06% of total T4 and free T3 0.2% of total T3.


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  • Radioimmunoassay.
  • Chemiluminescence.
  • Automated enzyme immunoassay.
  • Enzyme-linked immunosorbant assay (ELISA).
  • Competitive protein-binding assays.
  • Method determines required specimen type (plasma or serum).


  • Most veterinary commercial, reference or university laboratories.

Result Data

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Further Reading


Refereed papers

  • Recent references from PubMed and VetMedResource.
  • Singh A K et al (1997) Validation of non-radioactive chemiluminescent immunoassay method for analysis of thyroxine and cortisol in blood samples obtained from dogs, cat and horses. J Vet Diag Invest (3), 261-268.
  • Allen A L et al (1997) Effects of delayed serum separation and long-term storage on the measurement of thyroid hormones in equine blood samples. Vet Clin Pathol 26 (1), 10-13 PubMed.
  • Sojka J E & Levy M (1995) Evaluation of endocrine function. Vet Clin North Am Equine Pract 11 (3), 415-435 PubMed.
  • Dybdal N O et al (1994) Diagnostic testing for pituitary pars intermedia dysfunction in horses. JAVMA 204 (2), 627-632 PubMed.
  • Duckett W M et al (1989) Thyroid hormone periodicity in healthy adult geldings. Equine Vet J 21 (2), 125 PubMed.
  • Glade M J & Reimers T J (1985) Effects of dietary energy supply on serum thyroxine tri-iodothyronine and insulin concentrations in young horses. J Endocrinol 104, 93 PubMed.

Other sources of information

  • Duckett W M et al (1998) Thyroid Gland. In: Equine Internal Medicine. Eds: S M Reed S M & W M Bayly. W B Saunders Co, USA. pp 916-925.
  • Powell D et al (1997) The effect of feeding restricted diets on T4 and T3 concentrations in the exercising horse. Proc 15th Eq Nutr and Physiol Symposium. Fort Worth, TX. pp 33-38.
  • Allen A L et al (1993)C ongenital hypothyroidism dysmaturity and skeletal lesions in Western Canadian foals. Proc 39th Ann Conven Am Assoc Eq Pract. San Antonio, TX. pp 207-208.
  • Dybdal N O et al (1990) Endocrine Disorders. In: Large Animal Internal Medicine. Ed: B Smith. C V Mosby, USA. pp 1296-1302.