Endocrine
Hypothalamic-Pituitary Axis
The hypothalamic-pituitary axis is the interface between the nervous and the endocrine system.
The hypothalamus has extensive neural connections with with other regions of the brain and regulates many functions such as appetite control.
The hypothalamus contains two types of neurosecretory cells:
1. Neurohypophysial neurons
- Release vasopressin and oxytocin from nerve endings in posterior pituitary
- Traverse hypothalamic-pituitary stalk
2. Hypophysiotropic neurons
- Release hormones into the portal hypophysial vessels to regulate the secretion of tropic hormones from the anterior pituitary:
- Growth hormone releasing hormone (GHRH) and somatostatin from the hypothalamus affect the secretion of growth hormone (GH) from the anterior pituitary
- Dopamine from the hypothalamus affects the release of prolactin from the anterior pituitary
- Gonadotrophin releasing hormone (GRH) from the hypothalamus affects the release of luteinizing hormone (LH) and follicle stimulating hormone (FSH) from the anterior pituitary
- Thyrotropin releasing hormone (TRH) from the hypothalamus affects the secretion of thyoid stimulating hormone (TSH) from the anterior pituitary
- Corticotropin releasing hormone (CRH) from the hypothalamus affects the secretion of adrenocorticotropic hormone (ACTH) from the anterior pituitary.
The hypothalamic-pituitary axis is very vulnerable to damage from:
- Tumor growth/pressure effects
- Craniopharyngioma
- Hypothalamic/chiasmatic tumor
- Hydrocephalus and raised pressure in thrd ventricle
- Surgical trauma
- Radiation therapy (RT)
- Chemotherapy
Radiation therapy and the hypothalamus:
- There is evidence that the hypothalamus is more sensitive to RT than the pituitary
- After lower doses of RT (used for TBI and low dose prophylactic cranial RT) the primary site of damage is the hypothalamus (usually associated with isolated GH deficiency)
- Higher doses of RT (used for treatment of brain tumors) produce damage to the anterior pituitary with multiple pituitary hormone deficiencies
- The incidence of hypothalamic-pituitary deficiency after RT is time dependent
- The longer the time from RT, the higher the incidence of hormonal deficits
