Sexually dimorphic dynamics of thyroid axis activity during fasting in rats

Area of Science:

• Endocrinology
• Neuroendocrinology
• Reproductive Biology

Background:

• Starvation is known to induce hypothyroidism by affecting the hypothalamus-pituitary-thyroid (HPT) axis.
• A significant sexual dimorphism exists in the HPT axis response to starvation, with males exhibiting a more pronounced reaction than females.

Purpose of the Study:

• To elucidate the dynamics of the HPT axis response to fasting across multiple levels.
• To provide a more detailed description of the sexual dimorphism in the HPT axis response to starvation.

Main Methods:

• Analysis of paraventricular nucleus *Trh* expression in adult rats subjected to 24 and 48 hours of starvation.
• Measurement of serum thyroid-stimulating hormone (TSH) and thyroxine (T4) concentrations in fasted male and female rats.
• Assessment of thyrotropin-releasing hormone (TRH)-degrading ectoenzyme activity in the median eminence of fasted female rats.

Main Results:

• Inhibition of *Trh* expression and serum TSH/T4 concentrations occurred earlier and with greater intensity in male rats compared to females after 24 and 48 hours of starvation.
• In female rats fasted for 36-72 hours, serum TSH decreased after 36 hours, coinciding with increased TRH-degrading ectoenzyme activity.
• The temporal patterns of these HPT axis changes differed between sexes, suggesting sex-specific regulatory mechanisms.

Conclusions:

• The sexual dimorphism in TSH secretion kinetics during starvation is influenced by both central (paraventricular nucleus TRH neurons) and peripheral (TRH catabolism) factors.
• Enhanced TRH-degrading activity appears to be more sustained in males than females, contributing to the observed sex differences in HPT axis response to fasting.