Equine Hypothyroidism Study Guide

Overview and Clinical Importance

Hypothyroidism in horses is one of the most commonly diagnosed yet frequently misunderstood endocrine conditions in equine practice. True primary hypothyroidism is extremely rare in adult horses, with the most clinically significant presentations occurring in neonatal foals as congenital hypothyroidism and dysmaturity syndrome (CHD). This topic is critically important for the NAVLE because veterinarians must distinguish between true thyroid dysfunction and the far more common scenario of low thyroid hormone levels secondary to nonthyroidal illness syndrome (NTIS), also known as euthyroid sick syndrome.

Understanding the hypothalamic-pituitary-thyroid (HPT) axis and factors that influence thyroid hormone concentrations is essential for accurate diagnosis and appropriate clinical decision-making. The traditional clinical picture of an obese, laminitic horse being hypothyroid has been largely debunked, with these horses now recognized as suffering from equine metabolic syndrome (EMS) or pituitary pars intermedia dysfunction (PPID).

Thyroid Gland Anatomy and Physiology

Anatomical Location

The equine thyroid gland is a bilobed endocrine organ located dorsal to the trachea, just distal to the larynx. Each lobe measures approximately 2.5 cm × 2.5 cm × 5 cm and the total gland weighs approximately 0.04 g/kg body weight. The thyroid is not readily visible or palpable in normal horses, although it may become visible with aging or pathological enlargement (goiter).