Changes in thyroid hormone levels in chicken liver during fasting and refeeding

Area of Science:

• Endocrinology
• Comparative Physiology
• Nutritional Science

Background:

• Fasting alters circulating thyroid hormone levels in chickens, specifically increasing thyroxine (T4) and decreasing 3,5,3'-triiodothyronine (T3).
• The liver plays a crucial role in regulating thyroid hormone metabolism, with increased hepatic type III deiodinase implicated in T3 reduction during fasting.
• Mammalian studies suggest caloric restriction reduces tissue uptake of T4 and T3, prompting an investigation into this mechanism in chickens.

Purpose of the Study:

• To investigate whether liver uptake of thyroid hormones (T4 and T3) is affected by fasting in chickens.
• To compare hepatic thyroid hormone levels with plasma levels during fasting and refeeding periods.

Main Methods:

• Four-week-old chickens were subjected to a 24-hour starvation period followed by refeeding.
• Blood and liver samples were collected at various time points during refeeding.
• Thyroid hormone concentrations (T4 and T3) were measured in plasma and extracted liver tissue.

Main Results:

• Intrahepatic T4 levels were elevated, while T3 levels were decreased in fasted chickens compared to controls fed ad libitum.
• Plasma and hepatic levels of T3 and T4 showed parallel changes, indicating consistent availability within the liver.
• These findings suggest that liver T4 and T3 uptake is not diminished during fasting in chickens.

Conclusions:

• Nutritional status does not appear to affect thyroid hormone uptake by the chicken liver.
• Unlike in mammals, hepatic thyroid hormone transport is not a limiting factor in the observed hormonal changes during fasting in chickens.