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.


This article is available in full to registered subscribers

Sign up now to purchase a 30 day trial, or Login



  • 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

This article is available in full to registered subscribers

Sign up now to purchase a 30 day trial, or Login

Further Reading


Refereed papers

  • Recent references fromPubMedpublished during the last 12 months.
  • 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 Invest9(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 Pathol26(1), 10-13PubMed.
  • Sojka J E & Levy M (1995)Evaluation of endocrine function. Vet Clin North Am Equine Pract11(3), 415-435PubMed.
  • Dybdal N O et al(1994)Diagnostic testing for pituitary pars intermedia dysfunction in horses. JAVMA204(2), 627-632PubMed.
  • Duckett W M et al(1989)Thyroid hormone periodicity in healthy adult geldings. Equine Vet J21(2), 125PubMed.
  • Glade M J & Reimers TJ (1985)Effects of dietary energy supply on serum thyroxine tri-iodothyronine and insulin concentrations in young horses. J Endocrinol104, 93PubMed.

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. Philadelphia: W B Saunders Co. 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)Congenital 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, St Louis. pp 1296-1302.