Endocrine system. Central organs. Peripheral organs
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Endocrine system
The endocrine system consists of hormone-secreting cells and glands that regulate metabolism, growth, reproduction, stress responses, fluid-electrolyte balance, calcium balance, and circadian rhythm. Hormones enter the bloodstream and act on target cells with appropriate receptors.
1. Central endocrine organs
These organs coordinate endocrine activity, especially through the hypothalamic-pituitary axis.
| Organ | Main role and hormones |
|---|---|
| Hypothalamus | Links the nervous and endocrine systems. Produces releasing and inhibiting hormones that control the anterior pituitary: TRH, CRH, GnRH, GHRH, somatostatin, and dopamine. It also synthesizes ADH (vasopressin) and oxytocin, which are released from the posterior pituitary. |
| Pituitary gland (hypophysis) | Located below the hypothalamus. The anterior pituitary secretes GH, PRL, TSH, ACTH, FSH, and LH. The posterior pituitary stores and releases hypothalamic ADH and oxytocin. |
| Pineal gland | Secretes melatonin, helping regulate the sleep-wake cycle and circadian rhythms. |
2. Peripheral endocrine organs
These glands are largely regulated by pituitary hormones, circulating nutrient/mineral levels, or neural input.
| Organ | Principal hormones | Major function |
|---|---|---|
| Thyroid gland | T4 (thyroxine), T3, calcitonin | Regulates basal metabolic rate, growth, and development. |
| Parathyroid glands | Parathyroid hormone (PTH) | Maintains blood calcium and phosphate balance. |
| Adrenal glands | Cortex: cortisol, aldosterone, adrenal androgens. Medulla: epinephrine, norepinephrine | Stress response, blood pressure, salt-water balance, metabolism. |
| Endocrine pancreas - islets of Langerhans | Insulin, glucagon, somatostatin, pancreatic polypeptide | Regulates blood glucose and nutrient metabolism. |
| Ovaries | Estrogens, progesterone, inhibin | Menstrual cycle, pregnancy support, female sexual development. |
| Testes | Testosterone, inhibin | Male sexual development, spermatogenesis. |
| Placenta - during pregnancy | hCG, estrogen, progesterone, hPL | Maintains pregnancy and supports fetal development. |
| Thymus - mainly active in childhood | Thymic hormones | Supports maturation of T lymphocytes. |
Other endocrine tissues
Several non-classical organs also produce hormones:
- Kidneys: erythropoietin, renin, calcitriol
- Heart: atrial natriuretic peptide
- Liver: IGF-1, angiotensinogen
- Gastrointestinal tract: gastrin, secretin, cholecystokinin, incretins
- Adipose tissue: leptin
- Bone: osteocalcin and FGF23
Basic organization
Hypothalamus → Pituitary → Peripheral endocrine gland → Target-organ hormone
Most axes work through negative feedback. For example:
Hypothalamus (TRH) → anterior pituitary (TSH) → thyroid (T3/T4)
High circulating T3/T4 suppress further TRH and TSH release.
High circulating T3/T4 suppress further TRH and TSH release.
The list of classic endocrine glands and their principal hormones is consistent with the General Anatomy and Musculoskeletal System atlas, which includes pituitary, pineal, thyroid, parathyroids, adrenals, pancreatic islets, ovaries, testes, and placenta. The Merck endocrine overview also describes the hypothalamic-pituitary axis as the major regulatory link to many peripheral glands.
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