Sexually dimorphic dynamics of thyroid axis activity during fasting in rats

Starvation induces tertiary hypothyroidism in adult rodents. Response of the hypothalamuspituitary-thyroid (HPT) axis to starvation is stronger in adult males than in females. To improve the description of this sexual dimorphism, we analyzed the dynamics of HPT axis response to fasting at multiple levels. In adult rats of the same cohort, 24 and 48 h of starvation inhibited paraventricular nucleus Trh expression and serum concentrations of TSH and T4 earlier in males than in females, with lower intensity in females than in males. In adult females fasted for 36-72 h, serum TSH concentration decreased after 36 h, when the activity of thyrotropinreleasing hormone (TRH)-degrading ectoenzyme was increased in the median eminence. The kinetics of these events were distinct from those previously observed in male rats. We suggest that the sex difference in TSH secretion kinetics is driven not only at the level of paraventricular nucleus TRH neurons, but also by differences in post-secretory catabolism of TRH, with enhancement of TRH-degrading activity more sustained in male than female animals.