Effect of thyroidectomy upon seasonality in rams

Area of Science:

• Reproductive Endocrinology
• Animal Physiology

Background:

• Seasonal breeding is a key reproductive strategy in many mammals, including sheep.
• The role of thyroid hormones in regulating seasonal reproductive patterns in rams remains unclear.

Purpose of the Study:

• To investigate the effect of thyroidectomy on seasonal breeding patterns in male sheep (rams).
• To determine if thyroidectomy can overcome seasonal inhibition of reproductive activity.

Main Methods:

• Experiments involved thyroidectomized and intact rams exposed to controlled photoperiods (long/short days) or natural photoperiods.
• Measurements included plasma follicle-stimulating hormone (FSH) concentrations and scrotal circumference.
• Two experiments were conducted, with thyroidectomy performed in early autumn and spring.

Main Results:

• Thyroidectomized rams maintained high plasma FSH and scrotal circumference year-round, unlike intact rams which showed seasonal declines.
• Thyroidectomy in spring led to rapid increases in scrotal circumference and plasma FSH compared to controls.
• These effects were observed regardless of the photoperiod conditions.

Conclusions:

• Thyroidectomy abrogates seasonal breeding patterns in rams, indicating thyroid hormones are crucial for seasonal reproductive expression.
• The findings support a significant role for thyroid hormones in overriding seasonal reproductive inhibition in rams.