ISSN 2398-2942      

Thyroid: T3 assay


David Bruyette

Roger Powell

Synonym(s): Tri-iodothyronine


  • Thyrotropin releasing hormone (TRH, from the hypothalamus) stimulates production of TSH Thyroid stimulating hormone (TSH) from the anterior pituitary to regulate the synthesis and secretion of thyroxine (T4) and tri-iodothyronine (T3) . This is a negative feedback system, the released (free) T4 and T3 inhibiting further release of TSH (and possibly TRH).
  • Thyroxine (total thyroxine/tT4/T4) is the main secretory product of thyroid gland, as both free and protein bound forms. The vast majority is reversibly bound to carrier proteins, the minimal free unbound fraction (free T4/fT4/fT4eq/fT4d) being metabolically active.
  • Thyroxine (as a ‘prohormone’) is also converted to tri-iodothyronine (T3) by de-iodination for cellular effects.
  • T3 comprises 1-3% of circulating thyroid hormones and is also similarly and mostly protein bound. Only a minority (10-40%) of this though derives from the thyroid gland itself. The rest is converted/de-iodinated (extra-thyroidally) within the cells of the body.
  • The effects of (free) T3 are 3-5 times more potent than thyroxine, intra-pituitary conversion/production of T3 directly inhibiting TSH production.
  • Both free T3 (fT3/fT3eq/d) and total T3 (tT3/T3) can be measured, similarly to thyroxine (T4 and free T4) assays.
  • There is variable but seemingly less diurnal variation in [T3] than [T4] in dogs, but [T3] may be more affected in non-thyroidal diseases and less (detectably) reduced in hypothyroid cases.


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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).


  • Some more specialized commercial laboratories.


  • Varies with study, test methodology and reference method used, historically often relative to dynamic TSH stimulation testing.


  • 10%.


  • 79%.

Technique (intrinsic) limitations

  • Anti-T3 antibodies if present will often falsely increase values and are more commonly seen in hypothyroid cases (cf. anti-T4 antibodies and T4 measurements). Anti-T3 antibodies are estimated at 5-10% in some studies from all dogs tested, up to 34% in hypothyroid cases.
  • Analytical performance is often poorer (more imprecise) for low [T3] (cf equivalent T4 assays)

Result Data

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Further Reading


Refereed papers

  • Recent references from VetMedResource and PubMed.
  • Shiel R E, Brennan S F, Omodo-Eluk A J, Mooney C T (2007) Thyroid hormone concentrations in young, healthy, pretraining greyhounds. Vet Rec 161, 616-619 PubMed.
  • Panciera D L, Hinchcliff K W, Olson J, Constable P D (2003) Plasma thyroid hormone concentrations in dogs competing in a long-distance sled dog race. J Vet Intern Med 17, 593-596 PubMed.
  • Cowan L, Refsal K, Nachreiner R, Schoning P (1997) Thyroid hormone and testosterone concentrations in racing Greyhounds with and without bald thigh syndrome. J Vet Intern Med 11,142.

Other sources of information

  • Feldman and Nelson's (2015) Canine and Feline Endocrinology and Reproduction. 4th edn, Elsevier Science, USA.
  • Specific commercial laboratories offering T3 analysis in regard to testing, methodology and reference data.


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