Mild hyperthyroidism regulates the acute stress response in virgin female rats

Hyperthyroidism (HyperT) is a thyroid disorder affecting an estimated 0.2-1.3 % of the global population. The clinical implications of this pathology are significant, particularly when considering its close association with stress-related disorders that impact vulnerable populations, especially women at reproductive age. This study investigated the modulatory role of mild HyperT on the neuroendocrine stress response in virgin female Wistar rats. Two experimental groups were evaluated: a group with induced mild hyperthyroidism (HyperT) and a virgin control group (Control), each assessed under basal (non-stress) and acute stress conditions according to the corresponding experimental protocol. We evaluated the hormonal release (corticosterone and progesterone) induced by ether vapor inhalation and restraint stress in both experimental groups. Furthermore, we quantified the gene expression of receptors for glucocorticoids, progesterone, estrogen, and thyroid hormones, as well as the prolactin receptor and its downstream signaling pathway components in the medial basal hypothalamus (MBH) and hippocampus (HpC). Mild HyperT attenuates the ether stress-induced corticosterone release while responses induced by restraint stress were similar to controls. In the MBH, HyperT increased STAT5b and glucocorticoid receptor (GR) gene expression, and significantly decreased the progesterone receptor PRB/PRA ratio. In the HpC, HyperT increased the prolactin receptor (PRLR_L), STAT5b, and specific thyroid receptor isoforms (TRα2, TRβ2). Our findings demonstrate that mild hyperthyroidism differentially modulates the acute stress response depending on the stressor type, and may provide insight into the possible neurochemical mechanisms in key brain regions involved in the regulation of the stress response.