Hypothyroidism in Adult Equines

Hypothyroidism and Founder in Horses

by Robert N. Oglesby DVM

Introduction

Introduction » Anatomy and Physiology » Clinical Signs NOT Obesity or Founder » Diagnosis » The Truly Hypothyroid » More Info & Discussions

Hypothyroidism (HT) is a frequently misdiagnosed condition in adult horses that are overweight and have chronic founder. A sample measuring the two thyroid hormones, T3 and T4, is usually taken to confirm the diagnosis. The problem with this diagnosis is two fold:

  • Truly hypothyroid horses are unthrifty, have poor appetites, and are mildly depressed.
  • The blood levels of these hormones are quite variable in the healthy horse and do not accurately reflect thyroid function.
This article discusses the diagnosis, treatment, and prognosis of hypothyroidism in adult equines. Hypothyroidism has been well documented in newborn foals and for more information see: Hypothyroidism in Foals.

Anatomy and Physiology

Introduction » Anatomy and Physiology » Clinical Signs NOT Obesity or Founder » Diagnosis » The Truly Hypothyroid » More Info & Discussions

Thyroid hormone regulation is a complicated mechanism that starts in the brain. In response to numerous inputs the brain releases thyroid releasing hormone (TRH) that stimulates another part of the brain to release thyroid-stimulating hormone (TSH) into the blood where it circulates to the thyroid glands. There is a pair of them located on the back of the trachea, high in the neck close to the head.

Thyroid hormones, primarily T4, is released in response to stimulation from thyroid-stimulating hormone released by the brain. Tissues convert the T4 to the more active T3 form. These hormones affect the rate of many physiologic processes including: fat, protein and carbohydrate metabolism, increasing protein synthesis, increasing gluconeogenesis and promoting mobilization and utilization of glycogen stores.

Thyroid hormones also increase oxygen consumption, body temperature, heart rate and cardiac output, blood volume, enzyme system activity, and growth and maturity. Thyroid hormone is particularly important for adequate development of the central nervous system. While the exact mechanisms how thyroid hormones exert their effects are not well understood, it is known that thyroid hormones (primarily triiodothyronine) act at the cellular level. In humans, triiodothyronine (T3) is the primary hormone responsible for activity. Approximately 80% of T3 found in the peripheral tissues is derived from thyroxine (T4) which is the principal hormone released by the thyroid.

Clinical Signs NOT Obesity or Founder

Introduction » Anatomy and Physiology » Clinical Signs NOT Obesity or Founder » Diagnosis » The Truly Hypothyroid » More Info & Discussions

                       
You have just read the beginning of this article. To access the unabridged article become a Member of Horseadvice.com! Your membership gets you instant access to this and over 600 articles on our site. Other benefits of your membership include participation in our discussion boards and access to our one button PubMed search tool for each topic.

Horseadvice can teach you to be a more knowledgeable horse owner which leads to a healthier horse and save you money. Really, we guarantee it. Come Join Us!
Viewing topic 1 (of 1 total)
Viewing topic 1 (of 1 total)
  • You must be logged in to create new topics.