Are the thyroid and adrenal system alterations linked in depression?

Area of Science:

• Neuroendocrinology
• Psychiatry
• Chronobiology

Background:

• Major depressive disorder (MDD) is associated with dysregulation of the hypothalamic-pituitary-thyroid (HPT) and hypothalamic-pituitary-adrenal (HPA) axes.
• Glucocorticoids may suppress thyrotropin (TSH) and thyrotropin-releasing hormone (TRH) secretion, suggesting hypercortisolemia could impact HPT function.
• Existing data on HPA-HPT axis interactions in depression are inconclusive.

Purpose of the Study:

• To investigate the relationship between HPA axis activity and HPT axis function in major depression.
• To examine circadian rhythms and hormonal responses of the HPT and HPA axes in unmedicated patients with MDD compared to healthy controls.

Main Methods:

• Circadian rhythms of serum TSH and cortisol were measured every 4 hours over 24 hours in 145 unmedicated MDD patients and 25 healthy controls (HCs).
• TSH response to protirelin (TRH) stimulation was assessed at 0800h and 2300h.
• Cortisol response to the dexamethasone suppression test (DST) was evaluated.

Main Results:

• MDD patients exhibited reduced TSH levels (mesor and amplitude) and blunted TSH responses to TRH at 2300h compared to HCs.
• MDD patients showed increased cortisol levels (mesor) and impaired dexamethasone suppression.
• Cortisol levels and DST results were not correlated with TSH circadian parameters or TRH-induced TSH responses.

Conclusions:

• The findings do not support a significant role for hypercortisolemia in causing HPT axis dysregulation in major depression.
• While HPT and HPA axes are both disrupted in MDD, their interaction may not be driven by elevated cortisol levels impacting thyroid function.