Major Endocrine Hormones: Sources, Targets, and Effects

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Endocrine System Hormones

The endocrine system regulates physiological processes through the secretion of hormones. These chemical messengers are produced by specific glands and act on target tissues to maintain homeostasis, growth, metabolism, and reproduction.

Overview Table: Major Hormones, Sources, Targets, and Effects

Hormone	Organ that Secretes It	Target Tissue(s)	Main Effects
Antidiuretic hormone (ADH)	Posterior pituitary (produced in hypothalamus)	Kidneys	Promotes water reabsorption, reduces urine output, increases blood pressure
Oxytocin	Posterior pituitary (produced in hypothalamus)	Uterus, mammary glands	Stimulates uterine contractions during labor and milk ejection during breastfeeding
Thyroid-stimulating hormone (TSH)	Anterior pituitary	Thyroid gland	Stimulates secretion of thyroid hormones (T3 and T4)
Adrenocorticotropic hormone (ACTH)	Anterior pituitary	Adrenal cortex	Stimulates secretion of glucocorticoids (e.g., cortisol)
Growth hormone (GH)	Anterior pituitary	Most body tissues (especially bones and muscles)	Stimulates growth, cell reproduction, and regeneration
Prolactin	Anterior pituitary	Mammary glands	Promotes milk production
Melatonin	Pineal gland	Brain (suprachiasmatic nucleus), other tissues	Regulates circadian rhythms and sleep-wake cycles
Thyroxine (T4) and Triiodothyronine (T3)	Thyroid gland	Most body cells	Increase metabolic rate, regulate growth and development
Calcitonin	Thyroid gland	Bones	Lowers blood calcium levels by inhibiting bone resorption
Thymosin and Thymopoietin	Thymus	T lymphocytes (T cells)	Promote maturation of T cells for immune response
Parathyroid hormone (PTH)	Parathyroid glands	Bones, kidneys, intestines	Raises blood calcium levels by stimulating bone resorption, increasing calcium reabsorption in kidneys, and activating vitamin D
Cortisol	Adrenal cortex	Most body tissues	Increases blood glucose, suppresses immune response, helps body respond to stress
Aldosterone	Adrenal cortex	Kidneys	Promotes sodium and water reabsorption, increases blood pressure
Epinephrine and Norepinephrine	Adrenal medulla	Heart, blood vessels, liver, muscles	Initiate fight-or-flight response: increase heart rate, blood pressure, blood glucose
Insulin	Pancreas (beta cells of islets of Langerhans)	Most body cells (especially liver, muscle, fat)	Lowers blood glucose by promoting uptake and storage of glucose
Glucagon	Pancreas (alpha cells of islets of Langerhans)	Liver	Raises blood glucose by stimulating glycogen breakdown and gluconeogenesis

Key Hormones: Definitions and Functions

Antidiuretic Hormone (ADH)

Definition: A peptide hormone that regulates water balance in the body.
Secretion: Produced by the hypothalamus, released by the posterior pituitary.
Main Effect: Increases water reabsorption in the kidneys, reducing urine volume.
Example: ADH secretion increases during dehydration to conserve water.

Oxytocin

Definition: A peptide hormone involved in childbirth and lactation.
Secretion: Produced by the hypothalamus, released by the posterior pituitary.
Main Effect: Stimulates uterine contractions and milk ejection.
Example: Oxytocin is released during labor and breastfeeding.

Thyroid-Stimulating Hormone (TSH)

Definition: A glycoprotein hormone that stimulates the thyroid gland.
Secretion: Anterior pituitary.
Main Effect: Promotes secretion of thyroid hormones (T3 and T4).
Example: TSH levels rise when thyroid hormone levels are low.

Adrenocorticotropic Hormone (ACTH)

Definition: A peptide hormone that stimulates the adrenal cortex.
Secretion: Anterior pituitary.
Main Effect: Increases production of glucocorticoids (e.g., cortisol).
Example: ACTH is released in response to stress.

Growth Hormone (GH)

Definition: A protein hormone that stimulates growth and cell reproduction.
Secretion: Anterior pituitary.
Main Effect: Promotes growth of bones and muscles.
Example: GH is especially important during childhood and adolescence.

Prolactin

Definition: A protein hormone that stimulates milk production.
Secretion: Anterior pituitary.
Main Effect: Promotes lactation in mammary glands.
Example: Prolactin levels increase after childbirth.

Melatonin

Definition: A hormone that regulates sleep-wake cycles.
Secretion: Pineal gland.
Main Effect: Controls circadian rhythms.
Example: Melatonin secretion increases in darkness, promoting sleep.

Thyroxine (T4) and Triiodothyronine (T3)

Definition: Iodine-containing hormones that regulate metabolism.
Secretion: Thyroid gland.
Main Effect: Increase metabolic rate, influence growth and development.
Example: Deficiency leads to hypothyroidism; excess causes hyperthyroidism.

Calcitonin

Definition: A peptide hormone that lowers blood calcium levels.
Secretion: Thyroid gland.
Main Effect: Inhibits bone resorption by osteoclasts.
Example: Calcitonin is more active in childhood when bone growth is rapid.

Thymosin and Thymopoietin

Definition: Peptide hormones that promote T cell maturation.
Secretion: Thymus.
Main Effect: Essential for development of adaptive immunity.
Example: Thymus activity is highest in childhood.

Parathyroid Hormone (PTH)

Definition: A peptide hormone that raises blood calcium levels.
Secretion: Parathyroid glands.
Main Effect: Stimulates bone resorption, increases calcium reabsorption in kidneys, activates vitamin D.
Example: PTH is released when blood calcium is low.

Cortisol

Definition: A glucocorticoid hormone involved in stress response.
Secretion: Adrenal cortex.
Main Effect: Increases blood glucose, suppresses immune function.
Example: Cortisol levels rise during physical or emotional stress.

Aldosterone

Definition: A mineralocorticoid hormone that regulates sodium and water balance.
Secretion: Adrenal cortex.
Main Effect: Promotes sodium reabsorption in kidneys, increasing blood pressure.
Example: Aldosterone secretion increases when blood pressure is low.

Epinephrine and Norepinephrine

Definition: Catecholamine hormones involved in the fight-or-flight response.
Secretion: Adrenal medulla.
Main Effect: Increase heart rate, blood pressure, and blood glucose.
Example: Released rapidly in response to acute stress.

Insulin

Definition: A peptide hormone that lowers blood glucose.
Secretion: Pancreatic beta cells.
Main Effect: Promotes glucose uptake and storage.
Example: Insulin is released after eating to lower blood sugar.

Glucagon

Definition: A peptide hormone that raises blood glucose.
Secretion: Pancreatic alpha cells.
Main Effect: Stimulates glycogen breakdown and gluconeogenesis in the liver.
Example: Glucagon is released during fasting to maintain blood glucose.

Hormone Regulation: Feedback Mechanisms

Negative Feedback: Most hormone levels are regulated by negative feedback loops, where rising hormone levels inhibit further secretion.
Example: High blood glucose inhibits glucagon release and stimulates insulin secretion.

Summary Table: Hormone Classification

Hormone	Type
ADH, Oxytocin, GH, Prolactin, Insulin, Glucagon	Peptide/Protein
TSH, ACTH	Glycoprotein/Peptide
Thyroxine (T4), Triiodothyronine (T3)	Amino acid derivative
Cortisol, Aldosterone	Steroid
Epinephrine, Norepinephrine, Melatonin	Amino acid derivative

Additional info: Some hormone details (e.g., target tissues, effects) were inferred based on standard Anatomy & Physiology knowledge to provide a complete and academically useful study guide.