Triiodothyronine (T3), insulin and characteristics of 5′-monodeiodinase (5′-MD) in mare’s milk from parturition to 21 days post-partum.

The research article focuses on a study that measures the presence of specific hormones in mare’s milk and colostrum during the first 21 days after giving birth, and the potential roles of these hormones in the well-being of suckling neonates and mammary tissues.

Study Overview

• The research investigated the presence and concentration of two hormones — triiodothyronine (T3) and insulin — in mare’s colostrum and milk for the first three weeks after birth.
• The study also probed the activity of the enzyme 5′-monodeiodinase (5′-MD), which is instrumental in locally generating T3 in the mammary tissues.

Findings and Understanding

• After delivery, the study found that T3 increased to its highest concentration around the 4th day (1.14+/-0.08 nmol/L), declined till the 7th day and then stabilized to an average concentration of 0.71+/-0.06 nmol/L from days 7 to 21.
• The insulin level was highest on the day of birth (401.0 +/-24.9 microU/mL) and fell to the lowest point (25.0+/-3.4 microU/mL) by the 5th day. After this, it tended to increase gradually.
• The study demonstrated the presence of two types of 5′-MD in the mare’s milk: PTU-sensitive (type I) and PTU-insensitive (type II). Unlike the traditional type II 5′-MD, the one in mare’s milk was inhibited non-competitively by aurothioglucose.
• A strong correlation was found between T3 concentration and 5′-MD activity from the 1st to the 6th day of lactation, suggesting that T3 concentration in the milk might rely on the milk’s 5′-MD activity.
• The presence of type II 5′-MD implies that the local generation of T3 within the mammary glands might contribute to the lactation process.
• Assuming that a suckling foal consumes about 1.8 micrograms of colostral T3 (0.456 micrograms/L) daily, the study noted that T3 might have some influence within the foal’s intestinal tract.

Significance of Study

• This research is the first of its kind to present evidence of T3 and insulin concentrations, and T4-5′-MD activity in mare’s milk and colostrum. Understanding these hormones and enzyme activity could provide insight into the health and development of newborn foals.