Endocrinology is the study of the endocrine system (i.e., the hypothalamus, pituitary gland, thyroid gland, adrenals, and gonads), metabolic diseases, and certain aspects of nutritional medicine. The endocrine glands are responsible for producing and secreting hormones, which influence the function of cells in certain tissues of the body. Hormone secretion is controlled by highly regulated pathways, the most important of which is the hypothalamic-pituitary axis. The hypothalamus secretes and stores nontropic hormones (e.g., ADH, oxytocin) and releasing hormones (e.g., TRH, CRH, GnRH). The pituitary gland is composed of the anterior pituitary, which secretes tropic hormones (e.g., ACTH, TSH, FSH, LH) and whose function is controlled by hypothalamic releasing hormones, and the , which stores ADH and oxytocin. The gonads are the ovaries in female individuals and testicles in male individuals. Their function is controlled via the , as well as the secretion of (SHBG). Disruption of the hypothalamic-pituitary axis can result in the development of various endocrine disorders, which are classified according to the level of pathway disruption: primary (disorders of the peripheral endocrine gland), secondary (pituitary dysfunctions), and tertiary (hypothalamic disorders). An understanding of these hormone pathways is important for the diagnosis and management of endocrine disorders, particularly when interpreting changes in hormone levels and the results of suppression and/or stimulation tests.
For more information, see the articles "" and “ .”
Overview of endocrinological diseases
- Types of endocrinological diseases
- (See “” in “”)
- Diseases of the endocrine glands of the hypothalamic-pituitary axis
Basics of endocrinology
- Types of hormones
|Overview of the most important types of hormones|
|Type of hormone||Description||Example|
|Based on signaling pathways|
|Paracrine hormones|| |
|Autocrine hormones|| |
|Endocrine hormones|| |
|Based on chemical nature|
|Steroid hormones|| |
|Peptide hormones|| |
|Based on solubility|
Hydrophilic hormones (e.g., catecholamines) are stored in secretory granules and released when needed. Lipophilic hormones (e.g., adrenocortical steroid hormones) pass into the bloodstream once synthesized without being stored in cells.
- Degradation of hormones
Feedback control mechanisms
Hormone secretion is controlled by the following feedback mechanisms: 
- Hormone secretion by the endocrine gland suppresses the release of hypothalamic and pituitary hormones.
- Negative feedback loop types include:
- Ultrashort feedback loop: Hypothalamic hormones inhibit their own secretion via autocrine effects.
- Short feedback loop: Pituitary hormones inhibit the release of hypothalamic hormones.
- Long feedback loop: Hormones from peripheral endocrine glands inhibit the release of hypothalamic and pituitary hormones.
- Example: Thyroid hormone signaling decreases the production of (TRH) and (TSH).
- Positive feedback
Diagnosis of endocrine diseases
- Direct measurement of hormone blood levels (e.g., measuring prolactin blood level upon suspicion of )
- Stimulation of glands to detect underactivity (e.g., for Addison disease)
- Inhibition of glands to detect hyperactivity (e.g., for Cushing syndrome)
- Imaging of glands to determine:
- Specific laboratory studies (e.g., determination of thyrotropin receptor antibodies; HbA1c for diabetes mellitus)
- Regulation of hormonal secretion by the anterior pituitary gland via the
- Secretion/storage of ADH and oxytocin
- Reception and integration of sensory inputs (e.g., from area postrema, )
- Thirst and hunger regulation
- Autonomic function control
Pituitary gland (hypophysis)
- Located in the sella turcica (midline depression of the sphenoid bone) of the middle cranial fossa
- Connected to the hypothalamus via the pituitary stalk (a tube-like structure between the median eminence of the hypothalamus and the posterior pituitary that contains the axons of the posterior pituitary neurons)
- Consists of two major parts:
- The three major types of pituitary cells are:
- Acidophil cells
- Basophilic cells
- Do not secrete hormones 
- Stain poorly with both acidic and basic dyes
- Anterior pituitary gland
- Posterior pituitary gland: storage/release of ADH and oxytocin in pituicytes (glial cells)
- Intermediate pituitary gland: secretion of (MSH)
“B-FLAT”: Basophils secrete FSH, LH, ACTH, and TSH.
“PiG on Acid”: Prolactin and GH are secreted by Acidophils.
|Tropic hypothalamic hormones and their effects|
|Axis||Hypothalamus||Pituitary gland||Endocrine target organ|
|Hypothalamic-pituitary-adrenal axis|| |
|Hypothalamic-pituitary-thyroid axis|| |
|Nontropic hypothalamic hormones and their effects|
|Hypothalamic-pituitary-somatotropic axis|| |
|Hypothalamic-melanocortin system|| |
“No PRO-BLAM:” Derivatives of PROopiomelanocortin are Beta-endorphin, ACTH, and MSH.
|Hormones of the posterior pituitary gland|
|Hormone||Regulation ||Main effects||Clinical relevance|
|Antidiuretic hormone|| |
Hypothalamic and pituitary drugs
|Overview of hypothalamic and pituitary drugs|
|Drug class||Examples||Mechanism of action||Indications||Side effects|
|GnRH agonists|| || |
|GnRH antagonists|| |
|GHRH analogs || |
|GH receptor antagonists|| |
ADH antagonists 
| || || |
|ADH analogs|| |
- The adrenal cortex consists of three distinct layers:
- Feedback control mechanism
- See “ ” in “ .”
- The thyroid gland cells produce the following hormones: 
- The main function of the thyroid hormones is the regulation of basal metabolism and growth, while calcitonin is involved in calcium and phosphate homeostasis.
- Feedback control mechanism
- See “ .”
- Sex hormones
- Regulation: The secretion of sex hormones is controlled by the pituitary and hypothalamic hormones.
Physiological effects of sex hormones
Feedback control mechanisms
- Central regulation
- Peripheral regulation via feedback inhibition of GnRH, FSH, and LH secretion by the following hormones:
Sex hormone-binding globulins (SHBGs): a group of transporter proteins required for the transport of lipophilic sex hormones
- Hyperestrogenism (e.g., pregnancy, OCP use) causes increased SHBG levels.
- Female individuals with low levels of SHBG (e.g., PCOS) have higher levels of free testosterone, which is responsible for the development of hirsutism in these patients.
- Male individuals with high estrogen levels (e.g., in liver cirrhosis) are at risk of developing gynecomastia because of increased SHBG levels and decreased free testosterone levels.
- Integrating center: arcuate nucleus (hypothalamus)
- Projection: The arcuate nucleus relays signals to other hypothalamic nuclei via orexigenic and anorexigenic neurons.
- Clinical relevance
|Neuroendocrine regulation of appetite|
|Neurotransmitter/hormone||Regulation||Site of production||Effects|
|Endocannabinoid|| || |
|Neuropeptide Y|| || |
Regulation of satiety 
|Neuroendocrine regulation of satiety|
|Peptide YY|| || |
|Amylin || || |
|Drug class||Example||Mechanism of action||Indication||Side effects|
Cannabinoid receptor agonist 
|Synthetic progestin|| |
|GLP-1 agonist || |