Equine Hypothyroidism

Equine Hypothyroidism Overview

Also Known As (AKA):

• Thyroid Hormone Deficiency

Etiology:

• Primary Hypothyroidism: Rare in adult horses; potential causes include lymphocytic thyroiditis and idiopathic thyroid atrophy.

• Secondary Hypothyroidism: Resulting from pituitary or hypothalamic dysfunction; extremely uncommon in equines.

• Congenital Hypothyroidism: Occurs in foals born to mares with iodine imbalance (deficiency or excess) or ingestion of goitrogenic plants during gestation.

Pathophysiology:

• Insufficient production of thyroid hormones (T3 and T4) leads to a decreased basal metabolic rate, affecting multiple organ systems.

• In foals, maternal dietary iodine imbalance or exposure to goitrogens disrupts normal thyroid development and function, resulting in congenital hypothyroidism.

Clinical Signs:

• Adult Horses:

  • Lethargy

  • Exercise intolerance

  • Weight gain without increased appetite

  • Coat and skin changes (e.g., dry, coarse hair)

• Foals:

  • Musculoskeletal deformities

  • Delayed ossification

  • Prolonged gestation

  • Weakness at birth

  • Enlarged thyroid gland (goiter)

Laboratory Findings:

• Serum Thyroid Hormone Concentrations:

  • Decreased total thyroxine (tT4) and triiodothyronine (tT3) levels.

  • Free T4 by equilibrium dialysis (fT4d) may also be reduced.

• Thyroid-Stimulating Hormone (TSH):

  • Elevated in primary hypothyroidism; however, TSH assays are not routinely available for equines.

Diagnostic Imaging:

• Ultrasonography:

  • May reveal thyroid gland enlargement or structural abnormalities.

• Scintigraphy:

  • Assesses thyroid gland function and activity; limited availability in equine practice.

Confirmatory Diagnostic Tests:

• Thyrotropin-Releasing Hormone (TRH) Stimulation Test:

  • Evaluates thyroid gland responsiveness by measuring tT4 and tT3 levels before and after TRH administration.

• Thyroid Biopsy:

  • Histopathological examination to identify lymphocytic thyroiditis or other pathology.

Treatment:

• Thyroid Hormone Supplementation:

  • Levothyroxine sodium administered orally to normalize metabolic function.

• Dietary Management:

  • Ensure appropriate iodine intake; avoid goitrogenic substances.

Prognosis:

• Adult Horses:

  • Generally favorable with appropriate treatment; clinical signs typically resolve with hormone supplementation.

• Foals:

  • Prognosis depends on severity of musculoskeletal deformities; mild cases may improve with supportive care, while severe cases carry a guarded prognosis.

Additional Considerations:

• Nonthyroidal Illness Syndrome (Euthyroid Sick Syndrome):

  • Low thyroid hormone levels may result from concurrent illnesses; not indicative of true hypothyroidism.

• Misdiagnosis:

  • Hypothyroidism is often overdiagnosed in horses; thorough evaluation is essential to avoid unnecessary treatment.

References:

• Veterinary Merck Manual: Hypothyroidism in Animals

• Park City Equine: Thyroid Problems.

Useful Links:

• Equine Veterinary Journal. Diagnosis and management of thyroid disorders and thyroid hormone supplementation in adult horses and foals.

• College of Veterinary Medicine: Equine Thyroid Testing