Hormones: The Body's Chemical Messengers for Growth, Metabolism, and Reproduction

Hormones are signaling molecules produced by endocrine glands that travel via the bloodstream to regulate distant target organs. They orchestrate nearly every physiological process.

Major Endocrine Glands and Key Hormones

Gland	Key Hormones	Primary Functions
Hypothalamus	Releasing/inhibiting hormones (TRH, CRH, GnRH, GHRH, somatostatin, dopamine)	Controls pituitary; links nervous and endocrine systems
Anterior Pituitary	TSH, ACTH, FSH, LH, GH, Prolactin	Thyroid, adrenal, gonadal function; growth; lactation
Posterior Pituitary	ADH (vasopressin), Oxytocin	Water balance; uterine contraction, milk ejection
Thyroid	T3, T4, Calcitonin	Metabolic rate, growth, calcium homeostasis
Parathyroids	PTH	Calcium/phosphate regulation
Adrenal Cortex	Cortisol, Aldosterone, Androgens	Stress response, glucose, electrolytes, sex steroids
Adrenal Medulla	Epinephrine, Norepinephrine	Fight-or-flight
Pancreas	Insulin, Glucagon, Somatostatin	Glucose homeostasis
Gonads (Ovaries/Testes)	Estradiol, Progesterone, Testosterone, Inhibin	Reproduction, secondary sexual characteristics, bone
Pineal	Melatonin	Circadian rhythm
Adipose Tissue	Leptin, Adiponectin	Appetite, insulin sensitivity
Bone	Osteocalcin, FGF23	Energy metabolism, phosphate
Kidney	Erythropoietin, Renin, Calcitriol	RBC production, BP, calcium
Heart	ANP, BNP	Natriuresis, vasodilation
GI Tract	Gastrin, CCK, Secretin, GLP-1, Ghrelin	Digestion, appetite, glucose

Mechanisms of Action

• Peptide/Protein hormones: Bind surface receptors → second messengers (cAMP, IP3/DAG, tyrosine kinase).
• Steroid/Thyroid hormones: Diffuse through membrane → nuclear receptors → gene transcription.
• Amine hormones: Vary (catecholamines = surface; thyroid = nuclear).

Regulation: Feedback Loops

• Negative feedback: Target hormone inhibits upstream secretion (e.g., cortisol → ACTH/CRH).
• Positive feedback: Rare; e.g., estrogen → LH surge → ovulation; oxytocin → labor.

Hormonal balance is dynamic; dysregulation underlies diabetes, thyroid disease, infertility, osteoporosis, and more.