The hypothalamus controls the secretion of hormones from the pituitary gland, which in turn influences various endocrine glands throughout the body. The HPA can be divided into multiple sub-axes, depending on the specific endocrine target organs. The main components include:
- Hypothalamic-Pituitary-Adrenal (HPA) Axis
- Hypothalamic-Pituitary-Thyroid (HPT) Axis
- Hypothalamic-Pituitary-Gonadal (HPG) Axis
- Hypothalamic-Pituitary-Growth Hormone Axis
- Hypothalamic-Pituitary-Prolactin Axis
1. Hypothalamic-Pituitary-Adrenal (HPA) Axis
The HPA axis regulates the stress response and metabolism.
2. Hypothalamic-Pituitary-Thyroid (HPT) Axis
The HPT axis regulates metabolism, growth, and energy use.
3. Hypothalamic-Pituitary-Gonadal (HPG) Axis
The HPG axis controls reproductive functions.
4. Hypothalamic-Pituitary-Growth Hormone Axis
This axis regulates growth and development.
5. Hypothalamic-Pituitary-Prolactin Axis
This axis regulates lactation.
Key Concepts
- Negative Feedback Mechanism: Most of these axes operate via negative feedback to maintain homeostasis. For example, high levels of cortisol inhibit CRH and ACTH secretion.
- Interrelationships: The hypothalamus serves as a central integrator, linking the nervous and endocrine systems, responding to signals such as stress, circadian rhythms, and other stimuli.
Clinical Relevance
- HPA Dysregulation: Can lead to conditions like Cushing’s syndrome (hyperactivity) or Addison’s disease (hypoactivity).
- HPT Axis Disorders: Include hypothyroidism (low T3/T4) and hyperthyroidism (Graves’ disease).
- HPG Axis Abnormalities: Can lead to infertility, polycystic ovarian syndrome (PCOS), or hypogonadism.
- Growth Hormone Disorders: Deficiency leads to growth retardation (dwarfism), while excess causes acromegaly or gigantism.
- Prolactin Secretion: Excess prolactin can lead to galactorrhea and menstrual disturbances.