Endocrine glands and hormones
Endocrine glands have no ducts. They release their products, hormones, straight into the blood. Today a hormone is defined as a non-nutrient chemical that acts as an intercellular messenger and is made in trace amounts.
Nerves give fast, short-lived control. Hormones give slower but longer-lasting control. Together they coordinate the body. Invertebrates have simple endocrine systems; vertebrates have many glands.
Human endocrine glands: pituitary, pineal, thyroid, adrenal, pancreas, parathyroid, thymus, gonads (testis in males, ovary in females). The hypothalamus is part of the brain but also makes hormones. The gut, liver, kidney and heart also make hormones.
Hypothalamus
The hypothalamus is at the base of the forebrain. Its groups of neurosecretory cells make hormones that control the pituitary.
- Releasing hormones switch on pituitary secretion. Example: GnRH (gonadotrophin releasing hormone) makes the pituitary release gonadotrophins.
- Inhibiting hormones switch it off. Example: somatostatin stops the release of growth hormone.
These reach the anterior pituitary through a special blood link, the hypophyseal portal system. The posterior pituitary is controlled directly by nerves from the hypothalamus.
Pituitary gland
The pituitary sits in a bony hollow called the sella turcica and hangs from the hypothalamus by a stalk. It has two parts:
Adenohypophysis (anterior pituitary, pars distalis)
- Growth hormone (GH): growth of body.
- Prolactin (PRL): growth of mammary glands and milk production.
- TSH (thyroid stimulating hormone): makes the thyroid release its hormones.
- ACTH (adrenocorticotrophic hormone): makes the adrenal cortex release glucocorticoids.
- LH and FSH (gonadotrophins): in males, LH makes the testes release androgens and FSH (with androgens) controls sperm production; in females, LH causes ovulation and keeps the corpus luteum, FSH helps ovarian follicles grow.
- MSH (melanocyte stimulating hormone, from pars intermedia): controls skin pigment.
Neurohypophysis (posterior pituitary, pars nervosa)
It stores and releases two hormones made in the hypothalamus:
- Oxytocin: makes smooth muscles contract; strong contractions of the uterus during childbirth and release of milk.
- Vasopressin (ADH): makes the kidney tubules take back water, reducing urine.
Pineal, thyroid, parathyroid and thymus
- Pineal gland (top of forebrain): makes melatonin, which controls the 24-hour (diurnal) rhythm like sleep and wake, body temperature, and also affects metabolism, pigmentation, menstrual cycle and defence.
- Thyroid (two lobes on either side of the trachea joined by the isthmus; made of follicles): makes thyroxine (T4) and triiodothyronine (T3). Iodine is needed. They control basal metabolic rate (BMR), red blood cell formation, metabolism of carbs, proteins and fats, and water-salt balance. It also makes thyrocalcitonin (TCT), which lowers blood calcium.
- Parathyroid glands (four, at the back of the thyroid): make parathyroid hormone (PTH), which raises blood Ca2+ by releasing it from bone, increasing its reabsorption in kidneys and absorption from digested food. PTH is hypercalcaemic; TCT is hypocalcaemic.
- Thymus (between the lungs, behind the sternum): makes thymosins, which help T-lymphocytes mature (cell-mediated immunity) and antibody production (humoral immunity). It shrinks in old age, so immunity weakens.
Adrenal gland
One adrenal gland sits on top of each kidney. It has an outer cortex (three layers: zona reticularis, zona fasciculata, zona glomerulosa) and an inner medulla.
Adrenal medulla: 'emergency hormones'
Adrenaline (epinephrine) and noradrenaline (norepinephrine), called catecholamines, are released fast in stress or danger. They raise alertness, widen pupils, raise hair (piloerection), cause sweating, raise heart rate, force of beat and breathing rate, and break glycogen to raise blood glucose. They are the fight or flight hormones.
Adrenal cortex: corticoids
- Glucocorticoids (mainly cortisol): make new glucose (gluconeogenesis), break down fats and proteins, keep the heart and kidneys working, and suppress the immune response (anti-inflammatory). Cortisol also makes RBC production rise.
- Mineralocorticoids (mainly aldosterone): make the kidney tubules take back Na+ and water and remove K+ and phosphate. Keeps water-salt balance, blood volume and BP.
- Small amounts of androgens help hair growth on face, pubic and underarm areas at puberty.
Pancreas, testis and ovary
The pancreas is both exocrine (digestive juice) and endocrine. Its endocrine part is about 1 to 2 million Islets of Langerhans (1 to 2% of the pancreas).
- α-cells make glucagon: breaks liver glycogen into glucose (glycogenolysis) and makes new glucose. Raises blood sugar (hyperglycaemic). It reduces cellular glucose use.
- β-cells make insulin: helps liver cells and fat cells take in glucose and turns glucose into glycogen (glycogenesis). Lowers blood sugar (hypoglycaemic).
Testis (in the scrotum): Leydig (interstitial) cells make androgens, mainly testosterone. It controls growth of male sex organs, sperm formation, male features like beard, deep voice and muscular growth, and sexual behaviour (libido).
Ovary (in the abdomen): makes oestrogen (from growing follicles), which controls female sex organs, female features like high-pitched voice and breast growth, and female behaviour. After ovulation the follicle becomes the corpus luteum, which makes progesterone to support pregnancy and act on mammary glands for milk formation.
Hormones of the heart, kidney and gut
- Heart atria: atrial natriuretic factor (ANF) widens blood vessels and lowers blood pressure when it is high.
- Kidney (juxtaglomerular cells): erythropoietin (EPO) makes the bone marrow produce more RBCs.
- Gut (gastro-intestinal tract): gastrin (makes stomach glands release HCl and pepsinogen), secretin (makes the pancreas release water and bicarbonate), cholecystokinin (CCK) (makes the gall bladder and pancreas release bile and enzymes), and gastric inhibitory peptide (GIP) (slows stomach secretion and movement).
- Many other tissues make growth factors for normal growth and repair.
Mechanism of hormone action
A hormone works only on target cells that have its specific hormone receptor. Receptors are either on the cell membrane (membrane-bound receptors) or inside the cell (intracellular receptors, mostly in the nucleus). A hormone binds its receptor to form a hormone-receptor complex, which causes biochemical changes in the cell.
Types of hormones by chemistry
- Peptide, polypeptide, protein: insulin, glucagon, pituitary and hypothalamic hormones.
- Steroids: cortisol, testosterone, oestradiol, progesterone.
- Iodothyronines: thyroid hormones.
- Amino-acid derivatives: adrenaline.
Two ways of acting
- Through membrane receptors (mostly protein hormones): the hormone cannot enter the cell. It binds a receptor on the surface. This makes second messengers inside, like cyclic AMP (cAMP), IP3 or Ca2+, which change cell metabolism. The hormone is the first messenger.
- Through intracellular receptors (steroid and iodothyronine hormones): these are fat-soluble and pass through the membrane. The hormone-receptor complex acts on the genome, changing gene expression. The new proteins bring about the physiological change, like growth or development.
Hypo- and hyper-secretion disorders
| Hormone / gland | Too little (hypo) | Too much (hyper) |
|---|---|---|
| Growth hormone | Pituitary dwarfism (stunted growth) | In children: gigantism; in adults (middle age): acromegaly, a severe disfigurement of face, hands and feet, found late |
| ADH | Diabetes insipidus: lots of dilute urine, thirst, dehydration | Water retention |
| Thyroid hormones | Hypothyroidism: in pregnancy it causes poor growth of the baby, cretinism (stunted growth, mental retardation, low IQ, deaf-mutism); in adults, irregular menstrual cycle and low BMR. Simple goitre from iodine lack: thyroid swells | Hyperthyroidism: high BMR, weight loss, fast heart. Exophthalmic goitre (Graves' disease): enlarged thyroid, bulging eyeballs, weight loss |
| PTH | Low blood Ca2+ → tetany | Ca2+ drained from bones → weak bones, high blood Ca2+ |
| Adrenal cortex | Addison's disease: weakness, low BP, low sugar, bronze skin | Cushing's syndrome: fat face and trunk, high blood sugar |
| Insulin | Diabetes mellitus: high blood glucose (hyperglycaemia), glucose in urine, harmful ketone bodies | Hypoglycaemia (low blood sugar) |
Most of these can be managed by replacing the missing hormone (like insulin or thyroxine tablets) or by treating an overactive gland.
Key formulas and definitions
- Hypothalamus (releasing hormone) → anterior pituitary (tropic hormone) → target gland (hormone) → target cells
- Negative feedback: high target hormone ⟶ less releasing and tropic hormone
- Insulin: blood glucose ↓ (glycogenesis); Glucagon: blood glucose ↑ (glycogenolysis)
- PTH: blood Ca²⁺ ↑; Thyrocalcitonin: blood Ca²⁺ ↓
- Protein hormone → membrane receptor → second messenger (cAMP); Steroid → intracellular receptor → genes
Worked examples
1. Trace what happens when blood glucose rises after a meal.
High glucose → β-cells release insulin → liver and fat cells take in glucose and liver stores it as glycogen → blood glucose falls back to normal.
2. A person skips breakfast and blood glucose falls. Which hormone acts and how?
α-cells release glucagon → liver breaks glycogen into glucose and makes new glucose → blood sugar rises.
3. Why does lack of iodine cause the thyroid to swell?
Without iodine, little T4 is made. Low T4 does not switch off the hypothalamus and pituitary, so TSH stays high and keeps stimulating the thyroid, which enlarges: simple goitre.
4. A child is very tall for age and still growing fast. An adult's jaw, hands and feet slowly get larger. Explain both.
Both are hyper-secretion of growth hormone. Before bones stop growing it causes gigantism; in adults it causes acromegaly.
5. Insulin can't enter the cell, but testosterone can. Explain how each acts.
Insulin is a protein: it binds a membrane receptor and works through second messengers. Testosterone is a steroid: it crosses the membrane, binds an intracellular receptor and changes gene expression.
6. A patient has low blood calcium and muscle spasms. Which gland may be failing?
The parathyroid glands (less PTH). PTH normally raises blood Ca²⁺; without it Ca²⁺ falls and tetany occurs.
Common mistakes
- Thinking hormones act on every cell. They act only on target cells that have the matching receptor.
- Saying the posterior pituitary makes oxytocin and ADH. They are made in the hypothalamus and only stored and released by the posterior pituitary.
- Mixing up diabetes mellitus (less insulin, high blood sugar) with diabetes insipidus (less ADH, lots of dilute urine).
- Believing glucagon is the same as glycogen. Glucagon is a hormone; glycogen is stored sugar in the liver and muscles.