Endocrine System: Hormones and Glands – NDA Biology Notes

Your body has two communication systems. The nervous system uses electrical signals. They travel fast, reaching their target in milliseconds. The endocrine system uses chemical signals. These chemicals travel through the blood. They are slower than nerve signals, but they last longer, and they can reach every cell in the body simultaneously.

These chemical signals are called hormones. For the NDA exam, the endocrine chapter is a precision chapter. Questions are concentrated on a small, specific set: the three-hormone function table, the pancreas dual function, the iodine-thyroxine-goitre chain, the pituitary hormone portfolio, and the discovery of insulin. Every section below marks exactly where exam questions originate.

1. What is the Endocrine System?

The endocrine system is the body’s hormonal communication network. It consists of a set of glands, called endocrine glands, that produce and release chemical messengers (hormones) directly into the bloodstream, without ducts. This is what distinguishes endocrine glands from exocrine glands, which release their secretions through ducts.

The bloodstream carries hormones from the gland that produced them to target organs far away. The target organ has specific receptors that respond to that hormone, triggering a change in function or behaviour.

2. What are Hormones?

Hormones are chemical messengers produced by endocrine glands and released directly into the bloodstream. They act on target organs that may be far from the producing gland.

Chemical Nature of Hormones

Hormones are chemically diverse. They do not all belong to the same chemical category. All four types listed below are valid hormonal categories.

Chemical Category	Examples
Steroids	Cortisol, Oestrogen, Testosterone
Proteins or Peptides	Insulin, Growth Hormone, TSH, FSH, LH
Amino acid derivatives	Thyroxine (from tyrosine + iodine), Adrenaline (from tyrosine)
Fatty acid derivatives	Prostaglandins

★ IMPORTANT Hormones are NOT co-enzymes. Hormones act on target cell receptors: they signal cells to change their behaviour. Co-enzymes are small molecules that assist enzymes in catalysis, a completely different role. Hormones and co-enzymes must not be confused. This distinction has been tested directly.

3. The Pancreas: The Most Tested Endocrine Gland

The pancreas is the most frequently tested endocrine gland in this chapter. It has appeared across multiple NDA papers in different question formats.

Dual Function: Two Jobs in One Organ

The pancreas is unique. It is the only organ in the human body that functions as both an exocrine gland and an endocrine gland simultaneously.  [NDA 2007-II]

Function	Type	What It Does	Where It Goes
Produces digestive enzymes (lipase, amylase, proteases)	Exocrine	Digests food in the small intestine	Released into the duodenum through a duct
Produces insulin and glucagon	Endocrine	Regulates blood glucose levels	Released directly into the bloodstream

No other organ in the human body has this dual exocrine-endocrine function. The pancreas does not store bile. Bile is produced by the liver and stored in the gall bladder.  [NDA 2024-I]

Islets of Langerhans

Most of the pancreas produces digestive enzymes. Within the pancreatic tissue, small clusters of endocrine cells called the Islets of Langerhans produce the hormones. These islets are the specific cellular control centre for blood glucose regulation.  [NDA 2009-II | NDA 2006-I]

Cell Type	Hormone Produced	Effect on Blood Glucose
Beta (β) cells	Insulin	Lowers blood glucose (enables cellular uptake)
Alpha (α) cells	Glucagon	Raises blood glucose (stimulates liver glycogen breakdown)

The pancreas controls blood sugar, not the adrenal gland, not the parathyroid, not the spleen.  [NDA 2006-I | NDA 2006-II | NDA 2007-II]

4. Insulin: Lowering Blood Sugar

Insulin is the hormone that lowers blood glucose levels. When blood glucose is too high, after a meal for example, the beta cells in the Islets of Langerhans release insulin.  [NDA 2006-I | NDA 2006-III | NDA 2007-II]

Insulin works by enabling body cells to take up glucose from the blood. Glucose enters the cells. Blood glucose level falls. Without adequate insulin, glucose cannot enter most body cells. Blood glucose stays dangerously high. This is the basis of diabetes mellitus.

Insulin enables cellular glucose uptake. Growth hormone, TSH, and cortisol do not perform this function. Only insulin enables cells to absorb glucose from the blood.

Structure of Insulin

Insulin is a peptide hormone, made of two amino acid chains linked by disulfide bonds. Because it is a peptide, it cannot cross cell membranes. It acts on receptors on the cell surface.

• α-chain: 21 amino acids
• β-chain: 30 amino acids

Adult humans have two functional insulin genes, one encoding each chain.

5. Glucagon: Raising Blood Sugar

Glucagon works in the opposite direction to insulin. When blood glucose is too low, the alpha cells in the Islets of Langerhans release glucagon. Glucagon stimulates the liver to break down stored glycogen and release glucose into the blood. Blood glucose level rises.

Insulin and glucagon are antagonists. They push blood glucose in opposite directions. Together they maintain glucose balance (homeostasis). The liver is glucagon’s primary target organ.

6. Frederick Banting: Discovery of Insulin

Frederick Grant Banting discovered insulin, one of the most significant medical discoveries of the twentieth century.

This attribution must be precise. Joseph Lister pioneered antiseptic surgery using carbolic acid. Louis Pasteur developed germ theory. Edward Jenner introduced smallpox vaccination. None of these three discovered insulin. When an exam option offers Pasteur, Jenner, or Lister as the discoverer of insulin, it is always wrong.

7. The Thyroid Gland: The Iodine Chain

The thyroid gland is located at the front of the neck, shaped like a butterfly. Its entire story, from iodine intake to disease, forms one connected chain. Learning it as a chain is the most efficient way to retain it for the exam.

Thyroxin: The Metabolic Hormone

The thyroid produces thyroxin (also written thyroxine). Thyroxin controls the body’s metabolic rate: the speed at which cells use energy.

Thyroxin controls metabolic rate. It does NOT control blood glucose (that is insulin). It does NOT trigger ovulation (that is LH). It does NOT maintain pregnancy (that is progesterone).  [NDA 2019-II]

Thyroxin is an amino acid derivative, made from the amino acid tyrosine, chemically modified by the addition of iodine atoms.  [NDA 2025-I]

The Iodine Chain

Dietary iodine  →  absorbed into blood  →  thyroid concentrates iodine  →  uses iodine to make thyroxin  →  thyroxin released into blood  →  metabolic rate regulated

Iodised salt is the primary dietary source of iodine. This is why iodised salt is specifically important for thyroid health, not for any other gland.  [NDA 2025-I]

The thyroid is the only endocrine gland that stores iodine. No other gland, not the parathyroid, not the pituitary, not the adrenal, concentrates or stores iodine.  [NDA 2015-I]

The Iodine Confusion: Adrenaline Has No Iodine

A confusing question is asked about iodine’s role. The wrong option states that iodine helps the thyroid make adrenaline. This is completely wrong. Here is the corrected picture:

• Iodine → thyroid → thyroxin
• Tyrosine (without iodine) → adrenal medulla → adrenaline

Adrenaline is also made from the amino acid tyrosine, but iodine plays no role in adrenaline synthesis. The adrenal medulla makes adrenaline without any iodine.

8. Goitre and Cretinism

Goitre: When Iodine Is Deficient

When a person does not get enough dietary iodine, the thyroid cannot make sufficient thyroxin. The brain detects low thyroxin levels and sends signals telling the thyroid to work harder. The thyroid responds by growing, enlarging in an attempt to absorb more iodine from the blood.

This visible enlargement of the thyroid gland is called Goitre.  [NDA 2008-I | NDA 2015-I] It appears as a swelling at the front of the neck. The thyroid gland, not the adrenal cortex, not the adrenal medulla, not the pituitary, is the structure that enlarges.

Cretinism: Thyroxine Deficiency in Early Life

If thyroxine is severely deficient during foetal development and early childhood, the consequences are far more serious. The condition is called cretinism. It causes stunted physical growth and severe intellectual disability.  [NDA 2006-II]

Cretinism is caused by under-secretion of thyroxine. Not by adrenal hormone deficiency. Not by cortisone deficiency. Not by glucagon deficiency. Only thyroxine deficiency in early development causes cretinism.

★ IMPORTANT The complete thyroid deficiency chain: Insufficient dietary iodine → thyroid cannot make thyroxin → thyroid enlarges (Goitre) → if in early life → Cretinism (stunted growth + intellectual disability) Goitre = enlargement of thyroid (NOT adrenal, NOT pituitary) [NDA 2008-I | NDA 2015-I] Cretinism = thyroxine deficiency in foetal/early childhood (NOT adrenal, NOT glucagon deficiency) [NDA 2006-II]

9. The Pituitary Gland: Master of the Endocrine System

The pituitary gland is called the master endocrine gland. It sits at the base of the brain in a small bony cavity. It is a small gland but controls many other endocrine glands through the hormones it secretes.

The pituitary is unpaired. It exists as a single structure. Unlike the adrenal glands, testes, and ovaries, which come in bilateral pairs, there is only one pituitary gland.

Pituitary Hormone Portfolio

Hormone	Full Name	Function	Key Exam Fact
GH	Growth Hormone (Somatotrophin)	Controls overall body growth: height, muscle, organ development	Produced by pituitary: NOT adrenal, NOT thyroid, NOT pancreas
TSH	Thyroid-Stimulating Hormone	Signals the thyroid gland to produce thyroxin	Pituitary → TSH → thyroid → thyroxin (two-step chain)
FSH	Follicle-Stimulating Hormone	Stimulates development of egg follicles in the ovary	Part of the reproductive hormonal cycle
LH	Luteinizing Hormone	Triggers ovulation: LH surge = egg released from follicle [NDA 2007-I]	LH triggers ovulation: NOT FSH, NOT oestrogen, NOT progesterone

Menstrual Cycle: Controlled by Two Glands

The menstrual cycle in human females is controlled jointly by the ovary and the pituitary gland.  [NDA 2025-I]

The ovary produces oestrogen and progesterone. The pituitary produces FSH and LH. Both glands work together to regulate the 28-day cycle. Lacrimal glands produce tears, not hormones. Sebaceous glands produce skin oil, not reproductive hormones. Neither has any role in the menstrual cycle.

10. The Adrenal Gland: Two Layers, Two Hormones

The adrenal glands sit on top of the kidneys, one on each kidney. They exist as a bilateral pair. Each adrenal gland has two distinct layers with completely different functions.

Layer	Location	Hormone Produced	Key Function
Adrenal medulla	Inner layer	Adrenaline (epinephrine)	Fight-or-flight response: increases heart rate [NDA 2022-II]
Adrenal cortex	Outer layer	Cortisol	Stress response; suppresses inflammation; modulates metabolism

Adrenaline increases heart rate. This is its single most tested function.  [NDA 2022-II] Insulin does not increase heart rate (it lowers blood sugar). Melatonin does not increase heart rate (it regulates sleep). Thyroxin controls metabolic rate, not the same acute effect as adrenaline. Only adrenaline is the “fight or flight” emergency hormone that rapidly increases heart rate.

11. Three Hormones: Three Functions

Exams repeatedly present insulin, thyroxin, and adrenaline as options and ask which one performs a specific function. These three functions must be locked precisely and cannot be swapped.

★ IMPORTANT This is the most frequent source of confusion in the chapter. Insulin (Pancreas, β cells) → enables cells to absorb glucose from blood Thyroxine (Thyroid gland) → controls metabolic rate [NDA 2019-II] Adrenaline (Adrenal medulla) → increases heart rate [NDA 2022-II] These three functions are NOT interchangeable.

Hormone	Source	Single Most Tested Function	Wrong Associations to Reject
Insulin	Pancreas: Islets of Langerhans, β cells [NDA 2006-I | NDA 2009-II]	Enables cells to absorb glucose from blood (lowers blood sugar)	Does NOT increase heart rate. Does NOT control metabolic rate.
Thyroxin	Thyroid gland [NDA 2019-II | NDA 2025-I]	Controls metabolic rate [NDA 2019-II]	Does NOT control blood glucose. Does NOT trigger ovulation. Does NOT increase heart rate directly.
Adrenaline	Adrenal medulla [NDA 2022-II]	Increases heart rate (fight-or-flight) [NDA 2022-II]	Does NOT lower blood sugar. Does NOT control metabolic rate.

12. Other Endocrine Glands

Gland	Location	Hormone	Key Function
Parathyroid	Four tiny glands embedded behind the thyroid	Parathormone (PTH)	Regulates calcium and phosphate balance in the body
Thymus	Chest, behind the sternum	Thymosin	Maturation of T-lymphocytes (immune cells); most active in childhood, shrinks after puberty
Pineal	Small gland in the brain	Melatonin	Regulates sleep-wake cycle (circadian rhythm)

Melatonin from the pineal gland regulates sleep. It does NOT increase heart rate. That is adrenaline. The sleep-wake cycle distinction is directly tested as a contrast with adrenaline’s heart-rate function.

13. Gastrointestinal Hormones

Two hormones produced in the duodenum (small intestine) coordinate the digestive response.

Hormone	Produced By	Signals	Effect
Secretin	Duodenal lining	Pancreas	Releases alkaline pancreatic juice → neutralises acidic chyme from stomach
Cholecystokinin (CCK)	Duodenal lining	Gall bladder	Releases bile → fat emulsification in small intestine

Both hormones are released when acidic chyme arrives in the duodenum. Secretin and CCK are gastrointestinal hormones, produced in the gut wall, not in any traditional endocrine gland. They are covered in full in the Digestive System chapter (B5).

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