Equis ISSN 2398-2977

Endocrine: hyperthyroidism

Contributor(s): Ramiro Toribio, Han van der Kolk, Tim Watson

Introduction

  • Hyperthyroidism is a hypermetabolic disorder result of a high concentrations of free T3 and T4.
  • Few reports of hyperthyroidism in horses are documented.
  • Cause: excessive functional activity of the thyroid gland. It can be spontaneous or iatrogenic.
  • Signs:
    • Weight loss despite good appetitie.
    • Hyperexcitability.
    • Tremors.
    • Sweating.
    • Tachycardia.
    • Polyphagia.
    • Enophthalmos.
    • Alopecia.
    • Thyroid gland enlargement may be present, but not necessarily.
  • Diagnosis: T3 suppression test, measurement of free T4 concentration.
  • Treatment: (unilateral) thyroidectomy, potassium iodide.
  • Prognosis: good following hemithyroidectomy.

Pathogenesis

Etiology

  • The disease can be spontaneous, associated with malignancies (follicular adenocarcioma).
  • Hyperthyroidism can be iatrogenic from excessive iodide administration/exposure (Jod-Basedow phenomenon) or administration of thyroid hormones (thyrotoxicosis).
  • Auto-immune hyperthyroidism has not been documented in horses.
  • Most cases documented involved the right thyroid gland, but it is unlikely there is a predilection site.

Pathophysiology

  • The exact cause of the neoplasm is unknown.
  • Clinical signs are attributed to the elevated thyroid hormone concentrations.
  • Lesions in organs of neuroendocrine origin can be present (multiple endocrine neoplasia - MEN).
  • MEN as described in humans has been suspected but not documented in horses.
  • The main tumor associated with hyperthyroidism is adenocarcinoma.
  • Most thyroid adenomas are non-functional tumors.
  • The clinical signs are largely attributed to the elevated thyroid hormone concentrations.

Timecourse

  • Usually chronic, with gradual onset of signs over several years.
  • Short duration may be associated with iatrogenic hyperthyroidism.

Diagnosis

This article is available in full to registered subscribers

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

Treatment

This article is available in full to registered subscribers

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

Outcomes

This article is available in full to registered subscribers

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

Further Reading

Publications

Refereed papers

  • Recent references from PubMed and VetMedResource.
  • Breuhaus B A (2002)Thyroid-stimulating hormone in adult euthyroid and hypothyroid horses.J Vet Intern Med16, 109-115 PubMed.
  • Alberts M K, McCann J P & Woods P R (2000)Hemithyroidectomy in a horse with confirmed hyperthyroidism.JAVMA217, 1051-1054 PubMed.
  • Ramirez S, McClure J J, Moore R M, Wolfsheimer K J, Gaunt S D, Mirza M H & Taylor W (1998)Hyperthyroidism associated with a thyroid adenocarcinoma in a 21-year old gelding.J Vet Intern Med12, 475-477 PubMed.

Other sources of information

  • Toribio R E & Duckett W M (2004)Thyroid Gland.In:Equine Internal Medicine. Eds: Reed S M, Bayly W M & Sellon D C. 2nd edn. Saunders, St Louis. pp 1340-1356.


ADDED