Diagnosis and management of thyroid disorders and thyroid hormone supplementation in adult horses and foals.
Authors: Bertin François-René, Frank Nicholas, Breuhaus Babetta A, Schott Harold C, Kritchevsky Janice E
Journal: Equine veterinary journal
Summary
# Equine thyroid disorders remain diagnostically challenging due to the multifactorial regulation of the hypothalamic-pituitary-thyroid axis and horses' remarkable physiological tolerance of wide thyroid hormone fluctuations, yet thyroid hormone supplementation is frequently prescribed in clinical practice despite true hypothyroidism being exceptionally rare in adult horses. Benign thyroid tumours are relatively common in older horses, whilst primary hypothyroidism, hyperthyroidism, and congenital hypothyroidism occur infrequently; the critical distinction lies in recognising that low circulating thyroid hormone concentrations often represent a secondary adaptation to systemic illness, metabolic stress, or nonthyroidal illness syndrome rather than indicative of primary thyroid dysfunction. Interpreting serum thyroid hormone assays—including total and free T4 and T3 fractions—is complicated by pulsatile secretion patterns and numerous confounding factors, though dynamic testing such as thyrotropin-releasing hormone stimulation can provide valuable functional assessment of glandular responsiveness. This comprehensive review synthesises current understanding of equine thyroid pathophysiology across age groups, establishes evidence-based interpretation frameworks for blood hormone concentrations, and critically evaluates the clinical efficacy and justification of T4 supplementation in practice. Given the widespread prescription of thyroid supplementation for performance and obesity issues despite limited robust evidence supporting its use in truly euthyroid horses, this work is essential for practitioners seeking to refine diagnostic accuracy and avoid unnecessary pharmaceutical intervention.
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Practical Takeaways
- •Do not assume low thyroid hormone concentrations equal hypothyroidism in horses—consider nonthyroidal illness syndrome and other secondary causes before supplementing
- •Use dynamic testing (TRH stimulation) to help clarify thyroid function when blood hormone concentrations alone are ambiguous
- •Recognise that T4 supplementation is frequently used off-label for obesity and poor performance despite limited evidence supporting true primary hypothyroidism as the cause
Key Findings
- •Benign thyroid tumours are common in older horses, while primary hypothyroidism and hyperthyroidism are rare in adult horses and congenital hypothyroidism is rare in foals
- •Low blood thyroid hormone concentrations are often secondary responses to metabolic and disease states rather than true hypothyroidism
- •Dynamic testing such as thyrotropin-releasing hormone stimulation can help assess thyroid gland response to stimulation when interpreting results is challenging
- •Thyroid hormone supplementation is commonly used in equine practice despite true hypothyroidism being extremely rare