Felis ISSN 2398-2950

Thyroid: T3 suppression test

Synonym(s): Tri-iodothyronine suppression

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

Overview

  • Thyrotropin releasing hormone (TRH, from the hypothalamus) stimulates production of 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).
  • Using and mimicking this dynamic hypothalamic-pituitary-thyroid axis by exogenous suppression (eg oral T3 administration) effectively highlights any excessive and often independent production of T4 (and T3) by the thyroid gland. This suppression can be more sensitive than single hormonal measurements to confirm ‘hyper-production’ and clinical hyperthyroidism.
  • Thyroxine (total thyroxine/tT4/T4) is the main secretory product of thyroid gland, as both free and protein bound forms. Over 99% of thyroxine is reversibly bound to carrier proteins, their nature though less clear in cats (cf. dogs).
  • The minimal free unbound fraction (free T4/fT4/fT4eq/fT4d) is metabolically active and also converted to tri-iodothyronine (T3) for cellular effects.
  • T3 is also similarly and mostly protein bound, the effects of free T3 though 3-5 times more potent than thyroxine.

Sampling

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Tests

Methodologies

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

Availability

  • Most commercial laboratories (EIAs), fewer reference laboratories (RIAs).
  • In house total T4 measurement possible but as this test requires T3 measurement too, samples are best analyzed in the same laboratory.

Validity

  • Unclear, as without reference to a specific or traditional gold standard (such as scintigraphy), dynamic tests have largely been evaluated in contrasting feline populations, including those already diagnosed as hyperthyroid (based upon clinical presentation, palpable goitre, relative total [T4] and response to therapy).

Sensitivity

  • Unclear with no specific reference or gold standard diagnostic test.
  • Mean post [T4] in hyperthyroid cats (n=77) following suppression was 48.5 nmol/L, significantly higher than control groups.

Specificity

  • Unclear with no specific reference or gold standard diagnostic test.
  • Mean post [T4] in healthy (n=44) and non-thyroidal disease cats (n=22) respectively were 9.5 and 9.9 nmol/L, significantly lower than hyperthyroid cats.

Technique (intrinsic) limitations

  • Aside from the protocol itself, measurement assays are generally very good.

Technician (extrinsic) limitations

  • T3 dosing tablets are normally administered by owner and must be given appropriately (regularly and consistently).
  • Total [T3] should rise and [T4] fall following treatment if measured in the specified sampling window (2-4 h after final dose).

Result Data

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Further Reading

Publications

Refereed papers

  • Recent references from VetMed Resource and PubMed.
  • Paterson M E, Graves T K & Gamble D A (1990) Trilodothyionine T3 suppression test - and in the diagnosis of mild hyperthyroidism in cats. JVIM 4, 233-238.

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.


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