Disorders of the Hypothalamus and Pituitary Gland
Of the endocrine glands, the hypothalamus and pituitary glands are of major importance since they act as the coordinating centers of the endocrine system.
The hypothalamus is responsible for maintaining the body's internal balance (homeostasis) by stimulating or inhibiting major bodily functions such as the heart rate and blood pressure, body temperature, fluid and electrolyte balance, appetite and body weight, sleep cycle and function of the gastrointestinal track. The hypothalamus is also considered the master regulator of the endocrine system; Regulatory hormones secreted by the hypothalamus are transported by the hypophyseal-portal system to the anterior and posterior pituitary (Figure 2), prompting the release of secondary hormones that can affect various organ functions.
Figure 2. The hypophyseal-portal system
Hormones typically bind to membrane or nuclear receptors of specific target glands/organs, which in turn release hormones that exert negative feedback control on the releasing sites. (Figure 3).
Figure 3. Example of negative (in red) feedback inhibition:
The hypothalamus secretes releasing factors that act on the pituitary gland to stimulate the release of trophic hormones.
Trophic hormones act then on target organs (e.g., adrenal, thyroid or gonads), which in response produce other
hormones/signals, shutting down the production of releasing or/and trophic hormones.
The hypothalamus secretes various hormones that are then transported to the anterior pituitary (Figure 4):
- Corticotropin releasing hormone (CRH)
- Growth hormone releasing hormone (GHRH)
- Thyrotropin releasing hormone (TRH)
- Gonadotropin releasing hormone (GnRH)
- Somatostatin (inhibits growth hormone secretion)
- Prolactin releasing and prolactin inhibitor hormones
Figure 4. Overview of hypothalamic and pituitary hormones and their actions
The pituitary gland also secretes hormones in response to hypothalamic release hormones:
- Thyroid stimulating hormone (TSH)
- Adrenocortical stimulating hormone (ACTH)
- Follicle stimulating hormone (FSH)
- Luteinizing hormone (LH)
- Prolactin
- Melanocyte-stimulating hormone
- Endorphins
Supraoptic and paraventricular nuclei of the hypothalamus also secrete hormones that are transported to the posterior pituitary to be released into the circulation. These are:
- Anti-diuretic hormone (ADH): regulates plasma osmolality
- Oxytocin: regulates ejection of milk and uterine contractions.
Disorders of the hypothalamus can result in appetite, temperature and sleep disorders. As an example, hypothalamic obesity occasionally develops in response to major hypothalamic injury/damage affecting the centers of appetite regulation and energy balance. Hypothalamic obesity is characterized by an uninhibited eating disorder that often results in morbid obesity and can be associated with other obesity complications like diabetes, dyslipidemia, obstructive sleep apnea, mood disorder etc.
Disorders of the hypothalamus and/or anterior pituitary can also result in hypopituitarism, including adrenal insufficiency (see adrenal disorders section), hypothyroidism (see thyroid disorders section), hypogonadism (see puberty and its disorders section), growth hormone deficiency (see growth disorders section) and prolactin deficiency (inability to lactate).
Disorders of the posterior pituitary can result in diabetes insipidus and disorders pertaining to oxytocin deficiency (like inability to lactate, vaginal dryness, decreased libido etc.).
