Serum Thyroid Hormone and Thyrotropin Concentrations in Adult Horses as They Age.
Authors: Breuhaus Babetta A
Journal: Journal of equine veterinary science
Summary
# Editorial Summary Understanding how thyroid function naturally changes in ageing horses is increasingly important given that many animals remain in work well into their late teens and twenties; this study aimed to establish whether geriatric horses show the lower thyroid hormone concentrations and elevated thyrotropin (TSH) levels commonly associated with ageing in other species. Serum samples from 71 healthy horses across different age groups were analysed using validated radioimmunoassays to measure total and free thyroxine (T4), triiodothyronine (T3), and TSH, with additional thyrotropin-releasing hormone (TRH) stimulation testing performed in a subset. Peak total T4 concentrations occurred in 3–6-year-old horses and declined progressively with age, whilst serum TSH rose significantly in horses aged 15 years and older compared to younger groups; however, free T4 and T3 levels remained unchanged across all age groups. The TSH response to TRH stimulation was blunted in geriatric horses despite normal thyroid hormone responses, suggesting altered pituitary sensitivity rather than primary thyroid dysfunction. These findings indicate that age-related changes in serum TSH and total T4 are normal phenomena and should not be automatically interpreted as evidence of hypothyroidism in older horses—clinicians should exercise caution when interpreting borderline thyroid profiles in geriatric animals and consider free hormone concentrations and clinical signs before initiating treatment.
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Practical Takeaways
- •When evaluating thyroid function in aging horses, interpret elevated TSH in context of age rather than assuming pathology—this is a normal aging change with free T4 remaining adequate
- •Total T4 values naturally decline with age in healthy horses, so reference ranges should account for age rather than using universal standards
- •Clinical signs suggestive of hypothyroidism in geriatric horses warrant careful assessment as normal age-related changes in TSH and total T4 may not indicate true thyroid disease
Key Findings
- •Serum total T4 is highest in 3- to 6-year-old horses and declines with age, showing negative correlation with age
- •Serum TSH concentration significantly increases in horses ≥15-20 years old compared to younger age groups, with positive correlation to age
- •Free T4 and total/free T3 show no significant changes across age groups
- •TSH response to TRH stimulation is significantly different in older horses (≥15-20 years) compared to younger and intermediate age groups