The Endocrine System
Hormones, receptors, feedback control and the major endocrine glands
By the end of this unit you can…
- ✓Compare the endocrine and nervous systems and classify hormones chemically
- ✓Explain how water-soluble and lipid-soluble hormones act on target cells
- ✓Describe the three types of stimuli and the feedback control of hormone release
- ✓Describe the hypothalamus–pituitary relationship and each pituitary hormone
- ✓Describe the hormones of the thyroid, parathyroids, adrenals, pancreas, pineal and gonads
- ✓Relate hypo- and hypersecretion to classic endocrine disorders
Key terms
Practice →1Endocrine basics
The endocrine system coordinates and integrates the body with hormones — long-distance chemical messengers secreted into the blood. Compared to the nervous system, its responses are slower to start (seconds to days) but longer lasting. Hormones control reproduction, growth and development, electrolyte/water/nutrient balance, cellular metabolism and energy balance, and mobilization of body defenses.
Pure endocrine organs: pituitary, thyroid, parathyroid, adrenal and pineal glands. Organs with endocrine tissue among other functions: hypothalamus (a neuroendocrine organ), pancreas, gonads, placenta, thymus, and cells in the heart, kidneys, stomach, intestines, skin and adipose tissue.
| Chemical class | Solubility | Examples |
|---|---|---|
| Amino acid–based (amines, peptides, proteins) | Mostly water-soluble (except thyroid hormone) | Insulin, GH, ADH, oxytocin, epinephrine, norepinephrine |
| Steroids (from cholesterol) | Lipid-soluble | Gonadal hormones (estrogen, testosterone), adrenocortical hormones (cortisol, aldosterone) |
2How hormones act
Hormones only affect target cells that have the right receptors. Response depends on blood level of the hormone, number of receptors (up-regulation / down-regulation) and receptor affinity.
- 1Hormone (first messenger) binds a receptor on the plasma membrane.
- 2The receptor activates a G protein.
- 3The G protein activates the enzyme adenylate cyclase.
- 4Adenylate cyclase converts ATP to cyclic AMP (cAMP) — the second messenger.
- 5cAMP activates protein kinases, which phosphorylate proteins and change cell activity (amplification: one hormone → millions of product molecules).
Lipid-soluble hormones (steroids, thyroid hormone) diffuse through the plasma membrane, bind an intracellular receptor (in cytoplasm or nucleus), and the hormone–receptor complex binds DNA to turn on gene transcription → new proteins. Slower onset, longer effects. Lipid-soluble hormones travel in blood bound to transport proteins.
- Permissiveness
- One hormone can't exert full effects without another (thyroid hormone is needed for reproductive hormones to work normally).
- Synergism
- Combined effect of two hormones exceeds the sum (glucagon + epinephrine raising blood glucose).
- Antagonism
- One hormone opposes another (insulin vs glucagon).
3Control of hormone release
Blood hormone levels are controlled mainly by negative feedback. The stimulus for release can be:
| Stimulus | Example |
|---|---|
| Humoral — changing blood levels of ions or nutrients | Low blood Ca²⁺ → PTH; high blood glucose → insulin |
| Neural — nerve fibers stimulate the gland | Sympathetic nerves → adrenal medulla releases epinephrine |
| Hormonal — hormones from other glands | Hypothalamic hormones → anterior pituitary hormones → thyroid, adrenal cortex, gonads |
4Hypothalamus & pituitary
The pituitary gland (hypophysis) sits in the sella turcica, attached to the hypothalamus by the infundibulum. It has two lobes with different origins and different links to the hypothalamus:
- Posterior pituitary (neurohypophysis)
- Neural tissue. Stores and releases two hormones that are made by neurons in the hypothalamus and travel down axons (hypothalamic–hypophyseal tract): oxytocin and ADH.
- Anterior pituitary (adenohypophysis)
- Glandular tissue. Connected by the hypophyseal portal system — the hypothalamus secretes releasing and inhibiting hormones into it that control anterior pituitary secretion.
| Hormone | Source | Main target & effect |
|---|---|---|
| Oxytocin (OT) | Posterior (made in hypothalamus) | Uterine contractions in labor; milk ejection (let-down) — positive feedback |
| ADH (vasopressin) | Posterior (made in hypothalamus) | Kidneys — reabsorb water, concentrating urine; high levels constrict vessels |
| GH (growth hormone) | Anterior | Most cells, bone and muscle — growth; mobilizes fat; raises blood glucose; works via liver IGFs |
| TSH (thyroid-stimulating hormone) | Anterior | Thyroid → release of thyroid hormone |
| ACTH (adrenocorticotropic hormone) | Anterior | Adrenal cortex → glucocorticoids (cortisol) |
| FSH & LH (gonadotropins) | Anterior | Gonads — gamete production and sex hormone secretion; LH triggers ovulation |
| PRL (prolactin) | Anterior | Mammary glands — milk production |
5Thyroid & parathyroid glands
The butterfly-shaped thyroid gland in the anterior neck has two lobes joined by an isthmus. It is made of hollow follicles: follicular cells surround a colloid containing thyroglobulin, from which thyroid hormone (TH) is made using iodine. Parafollicular (C) cells between follicles make calcitonin.
- Thyroid hormone (T₄ thyroxine & T₃ triiodothyronine)
- The body's major metabolic hormone: increases basal metabolic rate and heat production (calorigenic effect), regulates growth and development (especially skeletal and nervous systems), maintains blood pressure. T₄ is mostly converted to the more active T₃ in tissues.
- Calcitonin
- Lowers blood Ca²⁺ (inhibits osteoclasts) — minor role in adults.
TH is regulated by negative feedback: hypothalamus (TRH) → anterior pituitary (TSH) → thyroid (TH). Rising TH inhibits TRH and TSH.
| Disorder | Cause | Signs |
|---|---|---|
| Hypothyroidism / myxedema (adult) | Low TH (often autoimmune — Hashimoto's thyroiditis) | Low metabolic rate, cold intolerance, weight gain, fatigue, puffy skin |
| Endemic goiter | Lack of dietary iodine → TSH keeps stimulating the gland | Enlarged thyroid |
| Cretinism | Hypothyroidism in infants | Short, disproportionate body; intellectual disability (preventable by newborn screening) |
| Graves' disease | Autoimmune antibodies mimic TSH → hyperthyroidism | High metabolic rate, sweating, weight loss, rapid heartbeat, nervousness; exophthalmos (protruding eyes) |
The parathyroid glands (usually four) on the posterior thyroid secrete parathyroid hormone (PTH) — the most important hormone controlling blood Ca²⁺. Low Ca²⁺ → PTH → (1) osteoclasts release Ca²⁺ from bone, (2) kidneys reabsorb more Ca²⁺ (and excrete phosphate), (3) kidneys activate vitamin D (calcitriol), which increases intestinal Ca²⁺ absorption.
6Adrenal glands
Each adrenal (suprarenal) gland sits on top of a kidney and is really two glands: an outer cortex (makes ~24 steroid hormones, the corticosteroids) and an inner medulla (nervous tissue; part of the sympathetic nervous system).
| Zone (outside → in) | Hormone class | Main hormone & effect |
|---|---|---|
| Zona glomerulosa | Mineralocorticoids | Aldosterone — kidneys reabsorb Na⁺ (and water follows) and secrete K⁺ → raises blood volume and BP. Stimulated mainly by the renin–angiotensin–aldosterone mechanism and high blood K⁺ |
| Zona fasciculata | Glucocorticoids | Cortisol — helps the body resist long-term stress: raises blood glucose (gluconeogenesis), mobilizes fat and protein, suppresses inflammation and the immune system. Controlled by ACTH |
| Zona reticularis | Gonadocorticoids | Weak androgens (DHEA) — in females contribute to sex drive and pubic/axillary hair |
| Medulla | Catecholamines | Epinephrine (~80%) and norepinephrine — short-term stress (fight-or-flight): ↑ heart rate, BP, blood glucose, bronchodilation |
7Pancreas & blood glucose
The pancreas is mostly exocrine (acinar cells make digestive enzymes), but scattered pancreatic islets (islets of Langerhans) are endocrine:
| Islet cell | Hormone | Effect on blood glucose |
|---|---|---|
| Alpha (α) cells | Glucagon | Raises it — liver breaks down glycogen (glycogenolysis) and makes glucose (gluconeogenesis) |
| Beta (β) cells | Insulin | Lowers it — cells take up glucose; glucose stored as glycogen and fat; the only hormone that lowers blood glucose |
| Type 1 diabetes mellitus | Type 2 diabetes mellitus | |
|---|---|---|
| Cause | Autoimmune destruction of β cells → no insulin | Insulin resistance (cells don't respond) ± reduced secretion |
| Onset | Usually childhood/young adults | Usually adults; linked to obesity and inactivity (~90% of cases) |
| Treatment | Insulin injections/pump | Diet, exercise, oral drugs, sometimes insulin |
The “three polys” of uncontrolled diabetes: polyuria (excess urine — glucose pulls water into urine), polydipsia (excess thirst) and polyphagia (excess hunger). Without insulin, fat breakdown produces ketones → ketoacidosis, a medical emergency.
8Other endocrine organs
| Organ | Hormone | Effect |
|---|---|---|
| Pineal gland | Melatonin | Sleep–wake cycle (rises at night); circadian rhythms |
| Ovaries | Estrogens & progesterone | Female sex characteristics, menstrual cycle, pregnancy |
| Testes | Testosterone | Male sex characteristics, sperm production, sex drive |
| Thymus | Thymosins, thymopoietin | Development of T lymphocytes |
| Heart (atria) | Atrial natriuretic peptide (ANP) | Lowers blood volume & BP — promotes Na⁺ and water loss |
| Kidneys | Erythropoietin (EPO); renin | Stimulates red blood cell production |
| Adipose tissue | Leptin | Signals satiety (fullness) to the brain |
| Stomach / small intestine | Gastrin, secretin, CCK, ghrelin | Regulate digestion; ghrelin stimulates hunger |
| Skeleton | Osteocalcin | Increases insulin secretion and sensitivity |
| Skin | Cholecalciferol (vitamin D₃) | Precursor of calcitriol → calcium absorption |