Influence of pregnancy on diurnal and seasonal changes in cortisol, T3 and T4 levels in the mare blood serum.

This study investigates how pregnancy affects the daily and seasonal changes in cortisol, T3 and T4 levels in the blood of mares.

Research Methodology

• The study involved four barren mares and four pregnant standard-bred mares that were kept and examined under the same conditions.
• The research team collected blood samples every 4 hours for one day each month throughout one year.

Findings and Analysis

• In barren mares, a daily rhythm was observed in cortisol and T3 levels, with peaks at specific times of the day depending on the season. The highest cortisol level was found at 0530 hrs in summer and 0830 hrs in winter, while the highest T3 level was found at 1330 hrs in summer and 1800 hrs in winter.
• In pregnant mares, a daily rhythm in cortisol level was only observed until the 5th month of pregnancy.
• A daily rhythm in T3 was found throughout pregnancy, with the peak always at 1400 hrs.
• No daily rhythm was observed in total protein content and in the T4 level.
• There were seasonal variations in T3 and T4 levels in both groups of mares. However, a seasonal change in cortisol level was only observed in pregnant mares.
• Pregnancy appears to eliminate the seasonal variation in total protein and introduces it in cortisol levels.
• Therefore, pregnancy in mares alters the daily rhythms as well as the seasonal cycles in the secretion and metabolism of these hormones.

The researchers concluded that these hormonal changes dependent on the time of day and season, could play critical roles in the physiological processes of pregnant mares. The precise role and mechanisms underlying these changes could be useful areas for future research.