Endocrine System: Glands and Hormones
Complete anatomy of the endocrine system - hypothalamus, pituitary, thyroid, parathyroid, adrenal, pancreas, pineal, and gonads. Hormone synthesis, regulation, and feedback.
This content is for informational purposes only. Always consult a healthcare professional.
Hypothalamus-Pituitary Axis
Hypothalamus
Nucleus
Hormone
Target
Paraventricular
TRH, CRH, oxytocin
Anterior pituitary, posterior pituitary
Supraoptic
ADH (vasopressin)
Posterior pituitary (kidney)
Arcuate
GHRH, dopamine (PIF)
Anterior pituitary
Preoptic
GnRH
Anterior pituitary
Ventromedial
Somatostatin (GHIH)
Anterior pituitary
Pituitary Gland (Hypophysis)
Feature
Description
Location
Sella turcica (sphenoid bone)
Size
1 × 1.5 × 0.5 cm
Weight
~0.5 g
Connection
Infundibulum (pituitary stalk) to hypothalamus
Blood supply
Superior/inferior hypophyseal arteries (from internal carotid)
Anterior pituitary (adenohypophysis):
Cell
Hormone
Target
Function
Somatotroph
Growth hormone (GH)
Liver, tissues
Growth, metabolism
Lactotroph
Prolactin (PRL)
Breast
Lactation
Corticotroph
Adrenocorticotropic (ACTH)
Adrenal cortex
Cortisol release
Thyrotroph
Thyroid-stimulating (TSH)
Thyroid
T3/T4 release
Gonadotroph
FSH, LH
Gonads
Reproduction
Posterior pituitary (neurohypophysis):
Stores and releases ADH and oxytocin (synthesized in hypothalamus)
Neural tissue (axons from paraventricular and supraoptic nuclei)
Thyroid Gland
Feature
Description
Location
Anterior neck (C5-T1), below thyroid cartilage
Weight
15-25 g
Lobes
2 (right, left) + isthmus
Blood supply
Superior thyroid (external carotid), inferior thyroid (thyrocervical trunk)
Innervation
Sympathetic (superior/middle cervical ganglia)
Follicular cells: Produce T3 (triiodothyronine) and T4 (thyroxine)
Parafollicular (C) cells: Produce calcitonin (↓ blood Ca²⁺)
Parathyroid Glands
Feature
Description
Number
4 (usually)
Location
Posterior to thyroid (2 superior, 2 inferior)
Size
3-5 mm each
Blood supply
Inferior thyroid artery
Chief cells: Parathyroid hormone (PTH) - ↑ blood Ca²⁺ (bone resorption, kidney reabsorption, vitamin D activation)
Adrenal (Suprarenal) Glands
Feature
Description
Location
Superior pole of each kidney
Weight
4-5 g each
Blood supply
Superior (inferior phrenic), middle (aorta), inferior (renal) arteries
Cortex (90%):
Zone
Hormone
Function
Zona glomerulosa (outer)
Aldosterone (mineralocorticoid)
Na⁺ retention, K⁺ excretion
Zona fasciculata (middle)
Cortisol (glucocorticoid)
Stress response, metabolism, immune suppression
Zona reticularis (inner)
DHEA, androgens (androstenedione)
Sex hormone precursors
Medulla (10%):
Chromaffin cells (modified postganglionic sympathetic neurons)
Epinephrine (80%) and norepinephrine (20%)
Stress response (fight or flight)
Pineal Gland
Feature
Description
Location
Roof of third ventricle (epithalamus)
Size
5-8 mm
Hormone
Melatonin
Function
Circadian rhythm regulation
Calcification
Pineal sand (visible on imaging, age-related)
Pancreatic Islets (Islets of Langerhans)
See digestive system for details. Distributed throughout pancreas (1-2% of pancreatic mass).
Gonads
Ovaries (Female)
Feature
Description
Location
Pelvis (lateral to uterus)
Hormones
Estrogen, progesterone, inhibin, relaxin
Function
Follicle development, ovulation, menstrual cycle
Testes (Male)
Feature
Description
Location
Scrotum
Hormones
Testosterone, inhibin
Function
Spermatogenesis, male secondary characteristics
Hormone Regulation
Feedback Loops
Type
Description
Example
Negative feedback
Output inhibits input (most common)
TSH → T4 → ↓ TRH/TSH
Positive feedback
Output amplifies input (rare)
Oxytocin during labor
Feed-forward
Anticipatory regulation
Insulin before meals
Hormone Types
Type
Examples
Mechanism
Peptide/protein
Insulin, GH, ACTH
Cell surface receptors (cAMP, tyrosine kinase)
Steroid
Cortisol, estrogen, testosterone
Intracellular receptors (gene transcription)
Amino acid derivative
T3/T4, epinephrine, melatonin
Both surface and intracellular