🩸 Endocrinology: Feedback Axes & Metabolic Homeostasis
Endocrine control relies on hierarchically nested negative feedback loops coupling neuroendocrine hypothalamic releasing factors, anterior pituitary tropins, and peripheral glandular target organs.
1. The Hypothalamic-Pituitary Axes
| Endocrine Axis | Hypothalamic Hormone | Anterior Pituitary Tropin | Target Gland & Effector Hormone | Negative Feedback Action |
|---|---|---|---|---|
| HPA Axis | Corticotropin-Releasing Hormone (CRH) | Adrenocorticotropic Hormone (ACTH) | Adrenal cortex (zona fasciculata) → Cortisol | Cortisol binds GR receptors in pituitary and hypothalamus, suppressing ACTH and CRH transcription. |
| HPT Axis | Thyrotropin-Releasing Hormone (TRH) | Thyroid-Stimulating Hormone (TSH) | Thyroid gland follicular cells → $T_3$ / $T_4$ | Free $T_3$ suppresses TSH $beta$-subunit and TRH release via thyroid hormone nuclear receptors. |
| HPG Axis | Gonadotropin-Releasing Hormone (GnRH, pulsatile) | Luteinizing Hormone (LH) & Follicle-Stimulating Hormone (FSH) | Gonads → Testosterone (Leydig cells) / Estradiol & Progesterone (Granulosa/Theca) | Steroids feedback negatively on GnRH pulse generator and pituitary gonadotropes (except mid-cycle estrogen positive feedback surge). |