Effect of food deprivation on baseline iodothyronine and cortisol concentrations in healthy, adult horses.

This study investigated how food deprivation impacts the levels of certain hormones and substances in the bloodstream of healthy, adult horses. It found that absence of food affected the horses’ thyroid and cortisol regulation, which partly recovered after refeeding.

Research Overview

• The research involved six healthy adult horses with normal baseline levels of certain blood factors: total triiodothyronine (T3), free T3 (FT3), total thyroxine (T4), free T4 (FT4), total reverse T3 (rT3), and cortisol.
• The horses’ normal thyroid function—what the researchers call ‘euthyroid’—was established based on their response to a standardized thyroid-stimulating hormone response test.

Methodology

• The researchers tested the serum concentrations of these substances before, during, and after a four-day period of food deprivation. Throughout this time, the horses were allowed to drink water freely.
• During this period, serum concentrations of T3, FT3, T4, FT4, rT3, and cortisol were measured every 24 hours.
• Coinciding tests were done 72 hours after food was returned to the horses.

Findings

• Food deprivation led to significant decreases in circulating T3, T4, FT3, and FT4 concentrations, and increases in rT3 and cortisol levels.
• Circulating T3, T4, FT3, and FT4 concentrations reached their lowest levels after 2 to 4 days of food deprivation, while rT3 and cortisol reached their highest levels after 1 and 2 days of food deprivation.
• Refeeding the horses resulted in a resurgence of serum T4 and FT4 concentrations, and a reduction in rT3 and cortisol levels towards their baseline values after a period of 72 hours.
• However, refuelling did not see a return of T3 or FT3 concentration to baseline values within the same window of time. Hence, T3 and FT3 concentrations were still affected following 72 hours after the horses resumed feeding.

Implications

• These results suggest that food deprivation has a significant impact on thyroid and cortisol regulation in horses.
• While some impacted blood factors partially recovered with the reintroduction of food, others remained disrupted even 72 hours post-refeeding, suggesting some effects of food deprivation may have longer-term impacts on these physiological systems.