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Hormones: Definition and Mechanism

Hormones are chemical messengers that travel through the bloodstream to regulate the activity of distant organs and tissues. They are essential for maintaining homeostasis, growth, metabolism, and reproduction.

• Hormones bind to specific receptors on or in target cells, triggering a physiological response.

• Hormonal effects can be rapid or slow, depending on the hormone and target tissue.

Hypothalamus and Pituitary Gland

Hypothalamus

The hypothalamus is a region of the brain that forms the floor and walls of the third ventricle. It plays a central role in regulating the endocrine system by controlling the pituitary gland.

• Integrates signals from the nervous system and initiates endocrine responses.

• Produces releasing and inhibiting hormones that regulate the anterior pituitary.

Pituitary Gland (Hypophysis)

The pituitary gland is a small, pea-sized gland suspended from the hypothalamus by a stalk called the infundibulum. It consists of two main lobes:

• Adenohypophysis (anterior pituitary): Arises from the pharynx; produces and secretes several hormones under hypothalamic control.

• Neurohypophysis (posterior pituitary): Arises from the brain; stores and releases hormones produced by the hypothalamus.

Hypothalamic-Pituitary Axis

The hypothalamus communicates with the anterior pituitary via a portal blood system, releasing hormones that stimulate or inhibit pituitary hormone secretion. The anterior pituitary produces tropic hormones that regulate other endocrine glands.

• Examples of hypothalamic hormones: Gonadotropin-releasing hormone (GnRH), Thyrotropin-releasing hormone (TRH), Corticotropin-releasing hormone (CRH).

• Examples of anterior pituitary hormones: Follicle-stimulating hormone (FSH), Luteinizing hormone (LH), Thyroid-stimulating hormone (TSH), Adrenocorticotropic hormone (ACTH), Prolactin (PRL), Growth hormone (GH).

Anterior Pituitary Hormones

Follicle Stimulating Hormone (FSH)

FSH is a gonadotropin that stimulates gamete production and sexual maturation.

• In females: Stimulates ovarian follicle development and estrogen production.

• In males: Stimulates sperm production in the testes.

• Disorders: Hypersecretion causes precocious puberty or early menopause; hyposecretion leads to delayed puberty or infertility.

Luteinizing Hormone (LH)

LH is a gonadotropin that triggers ovulation and stimulates sex hormone production.

• In females: Stimulates ovulation and production of estrogen and progesterone.

• In males: Stimulates testosterone production in the testes.

• Disorders: Hypersecretion causes premature sexual maturity; hyposecretion leads to infertility.

Thyroid Stimulating Hormone (TSH)

TSH stimulates the thyroid gland to produce thyroid hormones (T3 and T4), which regulate metabolism.

• Thyroid gland: Located below the larynx, consists of two lobes connected by an isthmus.

• Thyroid hormones increase basal metabolic rate, oxygen consumption, heart rate, and promote growth and development.

• Calcitonin: Secreted when blood calcium rises, promotes bone formation.

TSH Disorders

• Hyposecretion: Congenital hypothyroidism (cretinism), myxedema in adults (swelling, low metabolism, lethargy).

• Hypersecretion: Endemic goiter (iodine deficiency), Grave’s disease (autoimmune hyperthyroidism, exophthalmos, goiter).

Adrenocorticotropic Hormone (ACTH)

ACTH stimulates the adrenal cortex to produce glucocorticoids, mainly cortisol, which is involved in stress response, metabolism, and immune suppression.

• Adrenal cortex: Produces mineralocorticoids (aldosterone), glucocorticoids (cortisol), and sex hormones (androgens, estrogens).

• Adrenal medulla: Produces catecholamines (epinephrine, norepinephrine) for fight-or-flight response.

ACTH Disorders

• Hypersecretion: Cushing syndrome (excess cortisol, muscle/bone loss, fat redistribution, hypertension), Adrenogenital syndrome (androgen excess, early puberty, masculinization).

• Hyposecretion: Addison’s disease (bronzed skin, muscle weakness, low blood pressure, inability to handle stress).

Prolactin (PRL)

Prolactin stimulates milk production in the mammary glands and is involved in maternal behavior.

• Levels peak during pregnancy and after childbirth.

• Disorders: Hypersecretion causes inappropriate milk production, amenorrhea, impotence; hyposecretion leads to lack of milk production.

Growth Hormone (GH/hGH)

Growth hormone promotes growth of bones, muscles, and other tissues, especially during childhood and adolescence. It also stimulates protein synthesis and lipolysis.

• Stimulates production of insulin-like growth factors (IGFs) in the liver.

• Levels fluctuate with sleep, diet, and exercise.

GH Disorders

• Hypersecretion in childhood: Gigantism (excessive growth).

• Hypersecretion in adulthood: Acromegaly (thickening of bones in jaw, hands, feet).

• Hyposecretion: Pituitary dwarfism (short stature, normal proportions).

Posterior Pituitary Hormones (Neurohypophysis)

Oxytocin (OT)

Oxytocin is produced in the hypothalamus and released by the posterior pituitary. It stimulates uterine contractions during labor, milk ejection during breastfeeding, and may play a role in emotional bonding.

• Target organs: Uterus and mammary glands.

• Disorders: Hypersecretion causes early labor and overproduction of milk; hyposecretion leads to non-progression of labor and lack of milk let-down.

Antidiuretic Hormone (ADH, Vasopressin)

ADH is produced in the hypothalamus and released by the posterior pituitary. It promotes water reabsorption in the kidneys, reduces urine output, and increases blood pressure by constricting arterioles.

• Target organ: Kidneys.

• Disorders: Hyposecretion causes diabetes insipidus (excessive urination, dehydration); hypersecretion causes water retention and concentrated urine.

Regulation of the Pituitary: Feedback Mechanisms

Hormone secretion is tightly regulated by feedback mechanisms:

• Negative feedback: Increased hormone levels from target organs inhibit further release of tropic hormones from the pituitary (e.g., thyroid hormone inhibits TSH and TRH release).

• Positive feedback: Rare; e.g., stretching of the uterus increases oxytocin release, which further stimulates uterine contractions until delivery.

Summary Table: Major Anterior Pituitary Hormones

Key Equations

• Hormone feedback loop (example for thyroid axis):

• Renin-Angiotensin-Aldosterone System (RAAS) for fluid balance:

Additional info: This guide covers the structure and function of the endocrine system, major glands and hormones, feedback regulation, and common disorders. It is suitable for college-level Anatomy & Physiology students preparing for exams.