5'-deiodinase activity and circulating thyronines in lactating cows.

Area of Science:

• Veterinary Medicine
• Endocrinology
• Animal Physiology

Background:

• Thyroid hormones (T4, T3, rT3) are crucial for metabolism.
• Lactation significantly impacts physiological processes in dairy cows.
• Understanding thyroid hormone dynamics during lactation is vital for animal health.

Purpose of the Study:

• To investigate the correlation between lactation and thyroid hormone metabolism in dairy cows.
• To analyze changes in thyroid hormone levels and 5 deiodinase activity across different lactation stages.
• To explore the relationship between hypothyroid status and udder function during lactation.

Main Methods:

• Measured plasma and milk concentrations of total and free thyroxine (T4, fT4), triiodothyronine (T3, fT3), and reverse triiodothyronine (rT3).
• Assessed liver and mammary gland 5 deiodinase (5 D) activity in dry, early, middle, and late lactating cows.
• Compared thyroid hormone profiles and enzyme activity across lactation stages.

Main Results:

• Early lactation showed lower plasma T4 and rT3, and the lowest plasma T3 in the dry period.
• Liver 5 D activity decreased, while mammary 5 D activity increased during early lactation.
• Milk T4 and T3 concentrations decreased post-calving, while rT3 increased up to four months.

Conclusions:

• Lactating cows may experience a hypothyroid status.
• Organ-specific 5 deiodinase activity is rearranged to support udder function during lactation.
• Thyroid hormone metabolism is significantly altered to meet the demands of milk production.