Effect of altered thyroid hormone levels on hypothalamic-pituitary-adrenal function.

Area of Science:

• Endocrinology
• Neuroendocrinology
• Thyroid and Adrenal Axis Physiology

Background:

• Thyroid hormones significantly influence the hypothalamic-pituitary-adrenal (HPA) axis.
• The impact of short-term thyroid hormone deficiency on HPA axis responsiveness requires further elucidation.

Purpose of the Study:

• To investigate the effects of temporary thyroid hormone withdrawal on HPA axis function.
• To assess changes in adrenocorticotropic hormone (ACTH) and cortisol responses to ovine corticotropin-releasing hormone (oCRH) and metyrapone in hypothyroid patients.

Main Methods:

• Evaluated 10 athyreotic patients during thyroxine (T4) treatment and after T4 withdrawal (biochemical hypothyroidism).
• Measured plasma ACTH and cortisol responses to intravenous oCRH.
• Assessed plasma ACTH, cortisol, and 11-deoxycortisol responses to oral metyrapone.

Main Results:

• Serum TSH significantly increased, while total T4 and free T4 index decreased after T4 withdrawal.
• Peak and integrated ACTH and cortisol responses to oCRH were significantly greater after T4 withdrawal.
• ACTH response to metyrapone was not significantly different, and 11-deoxycortisol response was similar, suggesting maximal adrenal stimulation.

Conclusions:

• Short-term thyroid hormone deficiency enhances corticotroph sensitivity to oCRH.
• Hypothyroidism may reduce the ACTH response to metyrapone-induced hypocortisolemia.
• These findings suggest a potential decrease in hypothalamic CRH release during hypothyroidism.