DAT Review · Biology
The Endocrine System
On this page 7 sections
In 30 seconds
- The endocrine system uses hormones secreted into the bloodstream to regulate distant target cells. Know every major hormone: source gland, target tissue, and primary effect.
- Anterior pituitary (adenohypophysis) produces its own hormones (TSH, ACTH, GH, FSH, LH, prolactin). Posterior pituitary (neurohypophysis) stores/releases hormones made in the hypothalamus (ADH, oxytocin).
- Negative feedback is the dominant regulatory mechanism. The hypothalamic-pituitary-thyroid axis is the classic example: TRH → TSH → T3/T4 → negative feedback on TRH and TSH.
The college version
Core Review
Hormone Signaling Mechanisms
Hormones fall into two broad classes:
- Water-soluble (peptide/amino acid-derived): Cannot cross the plasma membrane. Bind cell-surface receptors that activate second messenger cascades (cAMP, IP3/DAG, Ca²⁺). Examples: insulin, glucagon, epinephrine, all hypothalamic/pituitary hormones.
- Lipid-soluble (steroid and thyroid hormones): Cross the plasma membrane and bind intracellular receptors (cytoplasmic or nuclear). The hormone-receptor complex acts as a transcription factor, directly regulating gene expression. Examples: cortisol, aldosterone, estrogen, testosterone, T3/T4.
Hypothalamic-Pituitary Axis
The hypothalamus is the master regulator, linking the nervous and endocrine systems. It controls the pituitary via two pathways:
- Anterior pituitary (adenohypophysis): Connected by a portal blood system. Hypothalamic releasing/inhibiting hormones travel through the hypothalamic-hypophyseal portal veins to stimulate or inhibit anterior pituitary hormone release.
- Posterior pituitary (neurohypophysis): Axons from hypothalamic neurons extend directly into the posterior pituitary. Hormones (ADH, oxytocin) are synthesized in hypothalamic cell bodies and stored/released from axon terminals in the posterior pituitary.
Master Hormone Table
| Hormone | Source | Target | Primary Effect(s) |
|---|---|---|---|
| TSH (thyroid-stimulating hormone) | Anterior pituitary | Thyroid gland | Stimulates T3/T4 synthesis and release |
| ACTH (adrenocorticotropic hormone) | Anterior pituitary | Adrenal cortex | Stimulates cortisol release |
| GH (growth hormone) | Anterior pituitary | Liver, bone, muscle, adipose | Stimulates growth (IGF-1), protein synthesis, lipolysis |
| FSH (follicle-stimulating hormone) | Anterior pituitary | Gonads | Ovarian follicle development (♀), spermatogenesis (♂) |
| LH (luteinizing hormone) | Anterior pituitary | Gonads | Ovulation, corpus luteum formation (♀), testosterone production (♂) |
| Prolactin | Anterior pituitary | Mammary glands | Milk production |
| ADH (antidiuretic hormone / vasopressin) | Hypothalamus, stored in posterior pituitary | Kidneys (collecting duct) | Water reabsorption (aquaporin insertion); vasoconstriction at high levels |
| Oxytocin | Hypothalamus, stored in posterior pituitary | Uterus, mammary glands | Uterine contraction (childbirth), milk ejection (let-down reflex) |
| T3/T4 (triiodothyronine/thyroxine) | Thyroid gland (follicular cells) | Most tissues | Increase basal metabolic rate, potentiate sympathetic nervous system |
| Calcitonin | Thyroid gland (C cells) | Bone, kidney | Decreases blood Ca²⁺ (minor role in adults) |
| PTH (parathyroid hormone) | Parathyroid glands | Bone, kidney, intestine (via vit D) | Increases blood Ca²⁺; decreases blood phosphate |
| Insulin | Pancreatic β cells (islets of Langerhans) | Most tissues (muscle, liver, adipose) | Decreases blood glucose (uptake, glycogenesis, lipogenesis) |
| Glucagon | Pancreatic α cells | Liver, adipose | Increases blood glucose (glycogenolysis, gluconeogenesis) |
| Cortisol | Adrenal cortex (zona fasciculata) | Most tissues | Stress response: increases blood glucose, suppresses immune system, protein catabolism |
| Aldosterone | Adrenal cortex (zona glomerulosa) | Kidneys (distal tubule, collecting duct) | Na⁺ reabsorption, K⁺ secretion, water follows — increases blood volume/pressure |
| Epinephrine (adrenaline) | Adrenal medulla | Heart, blood vessels, liver, lungs | Fight-or-flight: ↑ HR, bronchodilation, glycogenolysis, vasoconstriction (skin/GI) / vasodilation (skeletal muscle) |
| Estrogen | Ovaries (follicle), placenta | Reproductive tissues, bone | Female secondary sex characteristics, endometrial proliferation, bone density maintenance |
| Progesterone | Ovaries (corpus luteum), placenta | Uterus | Endometrial maintenance (secretory phase), pregnancy support |
| Testosterone | Testes (Leydig cells), adrenal cortex | Reproductive tissues, muscle, bone | Male secondary sex characteristics, spermatogenesis, anabolic effects |
Negative Feedback — The Thyroid Axis (Classic DAT Example)
- Hypothalamus secretes TRH (thyrotropin-releasing hormone) → anterior pituitary.
- Anterior pituitary secretes TSH → thyroid gland.
- Thyroid gland secretes T3 and T4 → target tissues (increase metabolism).
- Elevated T3/T4 inhibits further release of TRH and TSH (negative feedback). This keeps hormone levels within a narrow range.
Similar negative feedback loops exist for the HPA axis (CRH → ACTH → cortisol) and HPG axis (GnRH → FSH/LH → sex steroids). Disruption of negative feedback is the basis of many endocrine disorders (e.g., primary hypothyroidism → low T3/T4 → loss of negative feedback → high TSH).
Additional Hormone Mechanisms
- ADH and osmoregulation: When plasma osmolarity increases (dehydration), osmoreceptors in the hypothalamus trigger ADH release. ADH inserts aquaporin-2 channels in the collecting duct, increasing water reabsorption. Alcohol inhibits ADH → more urine production.
- Insulin and glucagon — antagonistic pair: After a meal, blood glucose rises → pancreatic β cells release insulin → glucose uptake, glycogenesis. During fasting, blood glucose falls → pancreatic α cells release glucagon → glycogenolysis, gluconeogenesis. Diabetes mellitus = insufficient insulin (Type 1) or insulin resistance (Type 2).
- Aldosterone and the RAAS system: Decreased blood pressure/blood volume → renin from kidneys → angiotensin II → aldosterone from adrenal cortex → Na⁺ retention → water retention → blood pressure increases.
Common Traps
- "Anterior pituitary makes ADH and oxytocin": No. The hypothalamus makes ADH and oxytocin. The posterior pituitary only stores and releases them. The anterior pituitary MAKES its own hormones (TSH, ACTH, GH, FSH, LH, prolactin).
- "T3 and T4 are water-soluble": They are amino acid derivatives (tyrosine) but act like lipid-soluble hormones — they cross the plasma membrane and bind nuclear receptors.
- "All pituitary hormones are tropic": GH acts on bone, muscle, and liver (not another endocrine gland). Prolactin acts on mammary glands. Neither is tropic.
- "ADH = aldosterone": ADH increases WATER reabsorption (aquaporins, collecting duct). Aldosterone increases Na⁺ reabsorption (and water follows osmotically). Different stimuli, different mechanisms.
Eli explains
The same idea, in plain words
Explain it like I’m 10
Your endocrine system is like your body's postal service. The hypothalamus is the main post office sorting center, sending special delivery instructions (releasing hormones) to the pituitary — which is the local post office that either forwards the letters or sends out its own. The thyroid, adrenals, pancreas, and gonads are neighborhood mailboxes delivering hormones through the bloodstream (the mail trucks) to specific addresses (cells with matching receptors). Negative feedback is like a thermostat: when the thyroid sends out enough T3/T4 (heat), the hypothalamus and pituitary get the message and turn down the furnace (reduce TRH and TSH). Insulin is the "store the sugar" hormone after you eat; glucagon is the "release the sugar" hormone when you're hungry. ADH is the "save water" hormone — like a city water conservation order during a drought.
Key takeaways
- Tropic hormones: Hormones that target other endocrine glands — TSH, ACTH, FSH, LH. GH and prolactin are NOT tropic (they act directly on non-endocrine targets).
- Portal system: The hypothalamic-hypophyseal portal system connects the hypothalamus to the anterior pituitary. The posterior pituitary uses neuronal connections, not portal blood.
- Pancreatic islet cells: β = insulin (decreases glucose); α = glucagon (increases glucose). Opposite effects, same organ.
- Lipid-soluble hormone mechanism: Bind intracellular receptor → hormone-receptor complex → transcription factor → alters gene expression. Slower but longer-lasting.
- Epinephrine vs. norepinephrine: Epinephrine (adrenal medulla, hormone) acts systemically. Norepinephrine (sympathetic postganglionic, neurotransmitter) acts locally. Structurally similar, different roles.
Check yourself
3 review questions from the chapter. Try each one, then open the answer.
A patient has elevated TSH and low T3/T4. Where is the defect most likely located, and why?
Show answer
The defect is most likely in the thyroid gland (primary hypothyroidism). When the thyroid cannot produce sufficient T3/T4, negative feedback on the anterior pituitary is lost, causing TSH to rise. If the defect were in the pituitary (secondary hypothyroidism), TSH would be LOW and T3/T4 would also be low.
How do the cell signaling mechanisms of insulin (a peptide hormone) and cortisol (a steroid hormone) differ?
Show answer
Insulin is water-soluble and cannot cross the plasma membrane. It binds a cell-surface receptor (tyrosine kinase), triggering a second messenger cascade (phosphorylation) that leads to cellular responses (e.g., GLUT4 translocation). Cortisol is lipid-soluble, crosses the membrane, and binds an intracellular receptor in the cytoplasm. The hormone-receptor complex translocates to the nucleus and acts as a transcription factor, altering gene expression. Insulin's effects are faster; cortisol's are slower but longer-lasting.
Which two hormones are released by the posterior pituitary, and where are they actually synthesized?
Show answer
ADH (antidiuretic hormone/vasopressin) and oxytocin are released by the posterior pituitary, but both are SYNTHESIZED in hypothalamic neurons (supraoptic and paraventricular nuclei). They travel down axons in the hypothalamic-hypophyseal tract and are stored in/released from axon terminals in the posterior pituitary.
Study tools & related lessonsYou’ll learn to · Related
You’ll learn to
- Identify the source gland, target tissue, and physiological effect of all major hormones.
- Distinguish between the anterior and posterior pituitary in terms of hormone production and hypothalamic control.
- Explain negative feedback using the hypothalamic-pituitary-thyroid axis as a model.
- Differentiate water-soluble (cell surface receptor) from lipid-soluble (intracellular receptor) hormone signaling mechanisms.
Sources & references
- OpenStax Biology 2e, Chapter 37: "The Endocrine System"
- NCBI Bookshelf, Endotext (NIH): "The Hypothalamic-Pituitary Axis"
- NIH National Institute of Diabetes and Digestive and Kidney Diseases: "Endocrine Diseases"
This lesson was adapted from the open educational references above; their licenses and attributions are preserved. See Copyright & Licensing.
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