Endocrine System Overview
A network of glands that communicates through chemical messengers called hormones.
The endocrine system is one of the body's two great communication systems (the other being the nervous system). It is made up of glands that secrete hormones directly into the bloodstream, which then travel to target cells with the matching receptors. Endocrine signaling is slower than nervous signaling but its effects last much longer.
Nervous System vs Endocrine System
- Messenger
- Nervous: Electrical impulses + neurotransmitters · Endocrine: Hormones (chemicals in blood)
- Speed
- Nervous: Very fast (milliseconds) · Endocrine: Slower (seconds to days)
- Duration
- Nervous: Brief · Endocrine: Longer-lasting
- Target
- Nervous: Specific muscles/glands · Endocrine: Any cell with the right receptor
The Major Endocrine Glands
- Hypothalamus
- Releasing/inhibiting hormones that control the pituitary; makes ADH & oxytocin
- Pituitarymaster gland
- Anterior: GH, TSH, ACTH, FSH, LH, prolactin · Posterior: stores ADH & oxytocin
- Thyroid
- T3, T4 (metabolism), calcitonin (lowers calcium)
- Parathyroids
- PTH (raises calcium)
- Adrenals
- Cortex: cortisol, aldosterone, androgens · Medulla: epinephrine, norepinephrine
- Pancreas
- Insulin (lowers glucose), glucagon (raises glucose)
- Ovaries / Testes
- Estrogen & progesterone / testosterone
- Pineal
- Melatonin (sleep-wake cycle)
Almost every endocrine gland has a matching set of "too much" and "too little" disorders that are heavily tested — hyper/hypothyroidism, Cushing's vs Addison's, SIADH vs diabetes insipidus, and diabetes mellitus. If you understand what each hormone normally does, you can reason out the signs and symptoms of its excess or deficiency rather than memorizing lists.
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The Pituitary Gland & Hypothalamus
The "master gland" — and the hypothalamus that directs it.
The pituitary gland sits at the base of the brain, just below the hypothalamus, which is its boss. The hypothalamus links the nervous and endocrine systems and sends releasing or inhibiting signals to the pituitary. The pituitary has two lobes with very different jobs.
Anterior Pituitary Hormones
The anterior pituitary makes and releases its own hormones (mnemonic: FLAT PiG).
- FSHfollicle-stimulating
- Ovaries/testes — egg & sperm development
- LHluteinizing
- Ovaries/testes — ovulation, testosterone
- ACTHadrenocorticotropic
- Adrenal cortex — release cortisol
- TSHthyroid-stimulating
- Thyroid — release T3 & T4
- Prolactin
- Breasts — milk production
- GHgrowth hormone
- Bones & tissues — growth and metabolism
Posterior Pituitary Hormones
The posterior pituitary does NOT make hormones — it stores and releases two hormones produced by the hypothalamus:
- ADH (antidiuretic hormone / vasopressin) — tells the kidneys to reabsorb water, concentrating urine. Too little = diabetes insipidus (floods of dilute urine); too much = SIADH (water retention, low sodium).
- Oxytocin — stimulates uterine contractions during labor and milk ejection ("let-down") during breastfeeding.
Notice how several anterior pituitary hormones end in "-stimulating" or "-tropic" (TSH, ACTH, FSH). These are tropic hormones — they don't act on the body directly; they tell another gland to release its hormone. So TSH → thyroid → T3/T4. This chain (hypothalamus → pituitary → target gland) is the backbone of how feedback works.
The Thyroid Gland
The body's metabolic thermostat.
The thyroid is a butterfly-shaped gland in the front of the neck, wrapped around the trachea. It produces two main hormones that set the body's metabolic rate.
Thyroid Hormones
- T3triiodothyronine
- The more active form; regulates metabolic rate, energy, heart rate, temperature
- T4thyroxine
- The main hormone released; converted to T3 in tissues. Requires iodine.
- Calcitonin
- LOWERS blood calcium by moving calcium into bone (opposite of PTH)
The Thyroid Control Chain
Hypothalamus releases TRH → anterior pituitary releases TSH → thyroid releases T3 & T4 → rising T3/T4 signals back to shut off TRH and TSH (negative feedback).
Too Much vs Too Little
- Hypothyroidismtoo little
- Metabolism slowed. Signs: Fatigue, weight gain, cold intolerance, bradycardia, constipation
- Hyperthyroidismtoo much
- Metabolism speeded up. Signs: Weight loss, heat intolerance, tachycardia, anxiety, diarrhea
The thyroid needs iodine to make T3 and T4. This is why iodine was added to table salt and why certain contrast dyes and medications interact with thyroid function. Understanding that thyroid hormone = metabolic rate lets you predict almost every sign of hypo- and hyperthyroidism on the exam.
? In primary hypothyroidism, the T3/T4 levels are low. Would you expect TSH to be high or low?
Parathyroid Glands & Calcium
Four tiny glands that guard blood calcium.
The four parathyroid glands are embedded on the back of the thyroid. They produce parathyroid hormone (PTH), the body's main regulator of blood calcium.
What PTH Does (it RAISES calcium)
- Bones — stimulates the release of calcium from bone into the blood
- Kidneys — increases calcium reabsorption (and activates vitamin D)
- Intestines — through activated vitamin D, increases calcium absorption from food
PTH vs Calcitonin — Opposites
- PTHParathyroids
- RAISES calcium (pulls it from bone)
- CalcitoninThyroid
- LOWERS calcium (deposits it into bone)
Calcium and phosphorus have an inverse relationship — when one goes up, the other goes down. PTH raises calcium and lowers phosphorus. This matters in chronic kidney disease, where phosphorus builds up, calcium falls, and PTH rises (pulling calcium from bones). Watch for signs of low calcium (tetany, tingling, positive Chvostek's and Trousseau's signs).
The Adrenal Glands
Two glands sitting on top of the kidneys — each with two distinct parts.
One adrenal gland sits on top of each kidney. Each has an outer cortex and an inner medulla that make completely different hormones.
Adrenal Cortex — the "3 S's"
- SaltMineralocorticoids
- Aldosterone — retains sodium & water, excretes potassium (raises BP); part of the RAAS
- SugarGlucocorticoids
- Cortisol — raises blood glucose, anti-inflammatory, the body's main stress hormone
- SexAndrogens
- Small amounts of sex hormones (e.g., DHEA)
Adrenal Medulla — Fight or Flight
- Produces the catecholamines: epinephrine (adrenaline) and norepinephrine
- Part of the sympathetic nervous system's "fight-or-flight" response
- Effects: increased heart rate, increased blood pressure, bronchodilation, raised blood glucose, dilated pupils
If cortisol/aldosterone are too high (Cushing's syndrome): high glucose, weight gain (moon face, buffalo hump), fluid retention, high BP, low potassium. If they are too low (Addison's disease): low glucose, weight loss, fatigue, low BP, high potassium, salt craving, bronze skin. Knowing cortisol = stress/sugar and aldosterone = salt lets you predict both.
The Pancreas & Blood Glucose
A dual-purpose organ that controls blood sugar.
The pancreas has two roles. Its exocrine part secretes digestive enzymes into the small intestine. Its endocrine part — clusters of cells called the islets of Langerhans — secretes hormones that control blood glucose.
The Islet Cells
- Beta cells
- Insulin — LOWERS glucose: moves glucose from blood into cells
- Alpha cells
- Glucagon — RAISES glucose: stimulates the liver to release stored glucose
- Delta cells
- Somatostatin — Inhibits both insulin and glucagon (regulates the balance)
How Blood Glucose Is Balanced
- After eating (glucose rises): beta cells release insulin → glucose moves into cells → blood glucose falls back to normal
- Between meals / fasting (glucose falls): alpha cells release glucagon → liver releases stored glucose → blood glucose rises back to normal
- Normal fasting blood glucose is about 70–100 mg/dL
In type 1 diabetes, the beta cells are destroyed and make little or no insulin — these clients require insulin to live. In type 2 diabetes, the body becomes resistant to insulin and/or the pancreas can't keep up. Without enough working insulin, glucose stays in the blood (hyperglycemia) and the cells "starve" — driving the classic signs of the 3 P's: polyuria, polydipsia, and polyphagia.
Hormone Regulation & Feedback
How the body keeps hormone levels from running away.
Negative Feedback (the main mechanism)
Most hormones are controlled by negative feedback — like a thermostat. When a hormone or the substance it controls reaches the target level, that signals the controlling glands to stop releasing more.
- Example: Low T3/T4 → hypothalamus releases TRH → pituitary releases TSH → thyroid releases T3/T4 → rising T3/T4 shuts off TRH and TSH.
- This keeps hormone levels stable within a narrow range.
Positive Feedback (rare)
In positive feedback, the response increases the original stimulus instead of shutting it off. It is uncommon in the body.
- Classic example: Oxytocin during labor — contractions push the baby's head against the cervix, which triggers more oxytocin and stronger contractions, until birth.
- Another example: the surge of LH that triggers ovulation.
Feedback explains lab patterns. In primary hypothyroidism, T3/T4 are low but TSH is high (the pituitary keeps shouting at a thyroid that can't respond). Recognizing "low hormone + high stimulating hormone = the target gland is failing" is a powerful exam shortcut.
Clinical Applications & Key Labs
Connecting the anatomy and physiology to the bedside.
Gland → Hormone → "Too Much/Too Little" Map
- Thyroid (T3/T4)
- Too much: Hyperthyroidism (Graves') · Too little: Hypothyroidism (myxedema)
- Posterior pituitary (ADH)
- Too much: SIADH (water retention, low Na+) · Too little: Diabetes insipidus (dilute urine)
- Adrenal cortex (cortisol)
- Too much: Cushing's syndrome · Too little: Addison's disease
- Parathyroid (PTH)
- Too much: Hyperparathyroidism (high Ca++) · Too little: Hypoparathyroidism (low Ca++, tetany)
- Pancreas (insulin)
- Too much: Hypoglycemia · Too little: Diabetes mellitus (hyperglycemia)
Key Endocrine Lab Values
Ranges vary by lab, facility, and textbook — for exams, use the range given in the question. See the full NCLEX-PN lab-value reference & sources →
- Fasting blood glucose
- 70–100 mg/dL — Pancreatic / insulin function
- Hemoglobin A1C
- < 5.7% normal; 5.7–6.4% prediabetes; ≥ 6.5% diabetes — Average glucose over ~3 months
- TSH
- ~0.4–4.0 mIU/L — Thyroid function (high in hypothyroidism)
- Serum calcium
- 9.0–11.0 mg/dL (ranges vary by lab) — Parathyroid function
- Serum potassium
- 3.5–5.0 mEq/L — Affected by aldosterone (Addison's = high; Cushing's = low)
You don't need to memorize every disorder in A&P — you need to know what each hormone normally does. Once you know that, "too much" and "too little" disorders, their signs, and their labs follow logically. The Med-Surg endocrine guide builds on this foundation to cover the disorders and nursing care in detail.
Frequently Asked Questions
Common student questions on this topic.
Why is the pituitary gland called the 'master gland'?
What is the difference between insulin and glucagon?
What do the thyroid hormones T3 and T4 do?
What is the difference between the adrenal cortex and the adrenal medulla?
What does parathyroid hormone (PTH) do?
What is a negative feedback loop in the endocrine system?
Quick-Reference Cheatsheet
Every key point from this guide on a single printable page.
Endocrine System Cheatsheet
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