Hormones and Endocrine Glands
Hormones are messenger molecules that are synthesized in one part of the body and move through the bloodstream to exert specific regulatory effects on another part of the body.
General functions of hormones
Major functions of hormones include:
- Responding to the internal and external environments:
- Physical stress (e.g., injury, exercise)
- Biochemical stress (e.g., ↓ blood sugar, hyperkalemia, hypercalcemia)
- Mental stress (e.g., fear/danger)
- Coordinating cellular and tissue responses between multiple organ systems
- Regulating body functions:
- Growth and development
- Biologic rhythms (e.g., circadian sleep cycles, menstrual cycle)
Endocrine glands and organs
The major endocrine glands in the body include:
- Pineal gland
- Thyroid gland
- Parathyroid gland
- GI system:
- Adrenal glands:
- Adrenal cortex
- Adrenal medulla
|Gland/organ||Hormone secreted by the gland/organ||Primary effect of the hormone|
|Hypothalamus||Thyrotropin-releasing hormone (TRH)||Stimulates pituitary thyrotropes to secrete thyroid-stimulating hormone (TSH)|
|Corticotropin-releasing hormone (CRH)||Stimulates pituitary corticotropes to secrete adrenocorticotropic hormone (ACTH)|
|Gonadotropin-releasing hormone (GnRH)||Stimulates pituitary gonadotropes to secrete FSH and LH|
|Growth hormone–releasing hormone (GHRH)||Stimulates pituitary somatotropes to secrete GH|
|Somatostatin||Inhibits the release of GH and TSH from the pituitary|
|Dopamine||Inhibits the release of prolactin from pituitary lactotropes|
|Anterior pituitary||TSH||Stimulates secretion of the thyroid hormones|
|Adrenocorticotropic hormone (ACTH)||Stimulates secretion of hormones by the adrenal cortex|
|Follicle-stimulating hormone (FSH)||Stimulates gamete production in the gonads|
|Luteinizing hormone (LH)||Stimulates gonadal androgen production|
|Growth hormone (GH)||Promotes the growth of body tissues|
|Prolactin (PRL)||Promotes the production of breast milk|
|Posterior pituitary||Antidiuretic hormone (ADH)||Stimulates water absorption by the kidneys|
|Pineal gland||Melatonin||Regulates sleep cycles|
|Thyroid gland||Thyroid hormones:||Stimulates cellular metabolism|
|Calcitonin||↓ Serum Ca2+|
|Parathyroid gland||Parathyroid hormone (PTH)||↑ Serum Ca2+|
|Adrenal cortex||Mineralocorticoids: aldosterone|
|Androgens||Stimulates secondary sex characteristics|
|Adrenal medulla||Catecholamines:||Stimulates fight-or-flight reaction|
|Estrogen and progesterone|
|Inhibin||Selectively inhibits the release of FSH|
|Placenta||Estrogen||Supports maternal physiology during pregnancy|
|Human chorionic gonadotropin (hCG)||Maintains the endocrine activity of the corpus luteum|
|Human placental lactogen (hPL)||Alters maternal insulin secretion in order to ↑ glucose for the fetus|
|Insulin-like growth factor||Regulates fetal growth|
|Placental CRH and glucocorticoids||Regulate organ development and maturation|
|Gland/organ||Hormone secreted by the gland/organ||Primary effect of the hormone|
|Stomach||Gastrin, histamine||Stimulates HCl secretion in the stomach|
|Serotonin||Stimulates gastric motility|
|Pancreas||Insulin||↓ Blood sugar levels by moving glucose across cell membranes into the intracellular space|
|Glucagon||↑ Blood sugar levels by stimulating gluconeogenesis and glycogenolysis|
|Gastric inhibitory peptide (GIP)||Stimulates insulin production|
|Thymus||Thymopoietin||Regulates immune function|
|Adipose tissue||Leptin||Suppresses food intake|
|Heart||Atrial natriuretic peptide (ANP)||Reduces plasma volume by stimulating diuresis|
|Liver||Angiotensinogen||A precursor for angiotensin II, a potent vasoconstrictor that stimulates aldosterone|
Types of Hormones
Amino acid–based hormones
Most hormones are amino acid–based.
- Hormones may be:
- Simple amino acid derivatives:
- Most commonly derived from tyrosine
- Example: epinephrine
- Short amino acid chains
- Example: insulin
- Long amino acid chains
- Example: epidermal growth factor
- Simple amino acid derivatives:
- May have a carbohydrate component → glycoproteins
- Dissolve in plasma:
- Travel freely to site of action
- Relatively shorter half-life as compared with steroid hormones
- Bind to receptors on the surface of the cell membrane
- Exception: thyroid hormones → amino acid–derived hormones that have intracellular receptors
- Synthesized from cholesterol → lipophilic
- Bound to plasma proteins in the blood:
- Proteins function as a reservoir.
- Relatively longer half-life, as compared with amino acid hormones
- Are able to move through the cell membrane’s lipid bilayer → steroid hormones bind to receptors within cells
- Primarily includes:
- Adrenal hormones
- Gonadal hormones
Most hormones bind to receptors, which then convey their message through secondary messengers and/or signal cascades. Steroid hormones, when bound to their receptors, are able to directly bind DNA and affect gene expression.
Modes of communication
There are several ways hormones send messages throughout the body:
- Endocrine hormones: released from specialized cells and travel through the blood to a distant site of action (true hormones)
- Paracrine hormones: released into the interstitial fluid to act on neighboring cells
- Autocrine hormones: Cells release hormones to regulate themselves.
- Neuroendocrine hormones: Neurons release a neurohormone that is carried through the blood to its distant site of action.
Signaling via plasma memrane hormone receptors
Plasma membrane receptors are typically required for amino acid–based hormones and use 2nd-messenger systems and signal cascades:
- 2nd messengers:
- A hormone binding to its receptor triggers release of the 2nd messengers, which then exert an effect within the cell.
- Typically small molecules (often not proteins)
- Common 2nd messengers include:
- Inositol-1,4,5-trisphosphate (IP3)
- Signaling cascades:
- Hormone binding to its receptor → triggers conformational change within the receptor → triggers a reaction in the next protein in the cascade → triggers a reaction in the next protein, and so forth
- Signal cascades may include:
- Sequential covalent modification of downstream proteins (often adding or removing phosphates to proteins)
- Release of 2nd messengers
- End results of signaling typically include:
- Altered gene expression
- Change of enzyme activities
Signaling via intracellular hormone receptors
Intracellular receptors generally cause direct gene activation:
- Steroid and thyroid hormones can diffuse directly into the cell.
- Bind to intracellular receptors
- Hormone-receptor complexes bind directly to hormone response elements within the DNA.
- Binding alters (typically stimulates) transcription of target genes → affects which proteins are being produced in the cell
Just about every hormone listed in the tables can be secreted in abnormal levels, resulting in a wide range of clinical conditions. Some of these conditions include:
- Hypopituitarism: condition characterized by a deficiency of all the pituitary hormones. Because the pituitary hormones regulate multiple organs, the effects of pituitary hypofunction are multisystemic. Causes of hypopituitarism include pituitary masses, congenital syndromes, trauma, infections, and vascular damage. Treatment includes hormonal replacement and addressing the underlying etiology.
- Acromegaly and gigantism: caused by excess production of pituitary GH. Typically, acromegaly is the result of excess GH after growth plate closure leading to large extremities and characteristic facies and gigantism is the tall stature seen in excess GH states in children before growth plate closure.
- Hyperprolactinemia: elevated levels of prolactin in the blood. The most common cause of hyperprolactinemia is a prolactin-secreting pituitary adenoma known as a prolactinoma. Presentations can include galactorrhea (milky discharge), oligomenorrhea, erectile dysfunction, and, in the case of large tumors, headaches and visual changes. Management typically involves dopamine agonists as 1st-line therapy, though surgery and/or radiation may be required.
- Central diabetes insipidus (DI): condition in which the kidneys are unable to concentrate urine due to a lack of circulating ADH. These low levels of ADH are due to either decreased production within the hypothalamus or decreased release from the posterior pituitary gland. Presentation is with polyuria, nocturia, and polydipsia. Central and nephrogenic DI are differentiated on the basis of measured ADH levels and response to the water deprivation test.
- Hyperthyroidism and hypothyroidism: abnormally high or low levels of thyroid hormone production in the thyroid gland. Individuals with these conditions will exhibit signs and symptoms related to the resulting elevated or suppressed cellular metabolism, including changes in energy level, weight, bowel movements, and heart rate.
- Diabetes mellitus (DM): ↓ insulin production in the pancreas or ↓ insulin sensitivity in peripheral tissues. Without normal insulin function, glucose is unable to be transported into tissues and remains trapped within the blood, leading to hyperglycemia. DM is treated with insulin-sensitizing agents or insulin itself.
- Cushing syndrome: condition resulting from chronic exposure to excess glucocorticoids. Etiologies include chronic glucocorticoid intake, increased adrenal secretion of cortisol, or increased pituitary or ectopic secretion of ACTH. Typical features include central obesity; thin, bruisable skin; abdominal striae; secondary hypertension; hyperglycemia; and proximal muscle weakness.
- Adrenal insufficiency: inadequate production of adrenocortical hormones (glucocorticoids, mineralocorticoids, and adrenal androgens). Management involves glucocorticoid and mineralocorticoid replacement therapy.
- Addison’s disease (primary adrenal insufficiency): due to pathology within the adrenal cortex itself. Etiologies include autoimmune disease, infections, and malignancy (among others).
- Secondary and tertiary adrenal insufficiency: due to the decreased production of ACTH within the pituitary, or hypothalamic disorders. These levels of insufficiency can also occur because of prolonged glucocorticoid therapy.
- Hypogonadism: condition characterized by reduced or no sex hormone production by the testes or ovaries. Hypogonadism may be due to failure of the gonads themselves or to defects in hypothalamic (GnRH) or pituitary (FSH/LH) stimulatory secretion. Symptoms include infertility, increased risk of osteoporosis, erectile dysfunction, decreased libido, and regression (or absence) of secondary sexual characteristics. Management is with hormone replacement therapy.
- Hyperparathyroidism: condition associated with elevated blood levels of PTH. Hyperparathyroidism may be due to inherent disease within the parathyroid gland or to abnormalities of calcium metabolism. Individuals with hyperparathyroidism classically present with stones (nephrolithiasis), bones (↓ bone mineral density), abdominal groans (nonspecific abdominal pain), and psychiatric overtones (neuropsychiatric symptoms). Management is typically surgical and treating any underlying conditions.
- Rickets and osteomalacia: disorders of decreased bone mineralization. Rickets affects the cartilage of the epiphyseal growth plates in children, whereas osteomalacia affects the sites of bone turnover in children and adults. These disorders are most commonly caused by vitamin D deficiency. Rickets commonly presents with skeletal deformities and growth abnormalities, whereas osteomalacia can present with bone pain, difficulty with ambulation, and pathologic fractures. Treatment includes vitamin D, calcium, and phosphorus supplementation.
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