A dietary deficiency of iodine leads to goitre because low thyroxine causes:

The pituitary to over-secrete TSH, enlarging the thyroid

The thyroid to make excess thyroxine

Insulin and glucagon are described as antagonistic because insulin lowers blood glucose while glucagon:

Raises blood glucose by breaking down glycogen

Also lowers blood glucose, but more slowly

Blood calcium level is kept constant by the antagonistic action of:

Calcitonin (lowers) and parathormone (raises)

Insulin (lowers) and glucagon (raises)

The pineal gland secretes melatonin, whose chief role is to regulate:

The sleep–wake (circadian) cycle

Why do both excess and deficiency of a hormone typically cause disease?

The body is tuned to a set-point by negative feedback, so departures either way disrupt it

Hormones can never be measured accurately

Only excess of a hormone ever causes disease

Hormones act only on the gland that makes them

Chemical Coordination and Integration — the Endocrine Glands, their Hormones and Disorders

This chapter is mostly a large, well-organised table to be learnt — gland, hormone, action, and the disorder of too much or too little. The way to keep it from becoming a blur is to learn each hormone as a pair of opposite disorders (hyposecretion versus hypersecretion), because that is exactly how NEET frames the questions: it names a symptom and asks for the hormone. Two conceptual anchors help. First, the pituitary is the "master gland" but is itself commanded by the hypothalamus — so the true master is the hypothalamus, and this hierarchy is examinable. Second, most hormone levels are held steady by negative feedback: a rising hormone shuts off the signal that produced it, which is why both excess and deficiency produce disease. Learn the table through these two ideas and it stops being rote.

The Hypothalamus, Pituitary and Thyroid

The hypothalamus links the nervous and endocrine systems, secreting releasing and inhibiting hormones that command the pituitary. So the pituitary is the "master gland", but the hypothalamus is master of the master.
The anterior pituitary secretes GH (growth), TSH (drives thyroid), ACTH (drives adrenal cortex), prolactin, FSH and LH. The posterior pituitary stores and releases oxytocin (childbirth, milk ejection) and ADH (water retention) — both actually made in the hypothalamus.
The thyroid makes thyroxine (T₄/T₃), which needs iodine and sets the body's metabolic rate. Its calcitonin lowers blood calcium. The thyroid is the classic gland for feedback and for deficiency disorders.

🎯 Why lack of iodine causes goitre — a feedback loop gone visible

Thyroxine is made from iodine, so a diet short of iodine cannot make enough thyroxine — hypothyroidism. Now the feedback loop takes over. Normally a healthy level of thyroxine tells the pituitary to ease off its stimulating hormone, TSH. But with thyroxine low, that brake is released, and the pituitary pours out ever more TSH trying to force the thyroid to produce. The thyroid, obeying the flood of TSH but lacking the raw material, enlarges without being able to make more hormone — and that swelling of the gland in the neck is goitre. So goitre is not the disease itself but the visible sign of the pituitary shouting at a thyroid that cannot answer. This is exactly why iodised salt prevents it, and why the same gland can enlarge whether the problem is too little iodine or an overactive drive — a subtlety NEET likes. In children, severe congenital hypothyroidism causes cretinism (stunted growth, intellectual disability); in adults it causes myxoedema.

The Pancreas and Adrenal Glands

The hormones that manage blood sugar and stress
Source	Hormone	Action
Pancreas — β cells	Insulin	Lowers blood glucose (drives uptake and glycogen storage)
Pancreas — α cells	Glucagon	Raises blood glucose (breaks down glycogen) — antagonist of insulin
Adrenal medulla	Adrenaline & noradrenaline	The "emergency hormones" — fight or flight; raise heart rate, BP, blood sugar
Adrenal cortex	Cortisol & aldosterone	Cortisol manages stress/metabolism; aldosterone reabsorbs Na⁺ and water

⚠️ The two diabetes are different diseases with a shared word

The word "diabetes" attaches to two unrelated conditions and NEET exploits the overlap. Diabetes mellitus is a disorder of insulin — either too little is made (Type 1) or the body responds poorly to it (Type 2) — so blood glucose stays high, spills into the urine (glycosuria), and the classic signs are excessive thirst, hunger and urination. Diabetes insipidus, met in the excretion chapter, is a disorder of ADH, has nothing to do with sugar, and produces large volumes of dilute urine. When a question mentions high blood sugar or glucose in urine, the hormone is insulin; when it mentions dilute urine with normal sugar, the hormone is ADH. Insulin and glucagon are also a textbook antagonistic pair — insulin lowers blood glucose, glucagon raises it — and questions often test which one you would expect to rise after a meal (insulin) versus during fasting (glucagon).

Other Glands and Feedback

The parathyroid makes PTH (parathormone), which raises blood calcium (the antagonist of the thyroid's calcitonin) — the pair that keeps blood calcium constant.
The testis makes testosterone (male secondary characters, sperm) and the ovary makes oestrogen and progesterone (female characters, menstrual cycle, pregnancy). The pineal makes melatonin, regulating the sleep–wake cycle.
The thymus secretes thymosin, maturing the T-lymphocytes for immunity; it is large in children and shrinks with age. Several other tissues (heart → ANF, kidney → erythropoietin) also act as endocrine sources.

ℹ️ Negative feedback: why hormones almost never run away

Most hormones are held within a narrow range by negative feedback, and understanding it explains a whole column of the disorder table at once. The logic is a thermostat: when a hormone (or the substance it controls) rises high enough, it switches off the signal that was producing it, and when it falls, the brake is released and production resumes. Blood calcium is the clean example — a rise triggers calcitonin to lower it, a fall triggers PTH to raise it, and the two opposing hormones hold the level steady between them. The same loop runs the thyroid (thyroxine feeds back on TSH) and blood sugar (insulin and glucagon). The practical consequence, and the exam point, is that both too much and too little of a hormone cause disease, because the body is tuned to a set-point and departures in either direction disrupt it. That is why the disorder table always has two rows per hormone, hyper and hypo, rather than one.

Key takeaways

The pituitary is the "master gland", but the hypothalamus commands it — so the true master is the hypothalamus.
Posterior pituitary only stores oxytocin and ADH (made in the hypothalamus); anterior secretes GH, TSH, ACTH, FSH, LH, prolactin.
Thyroxine needs iodine; iodine lack → low thyroxine → high TSH → thyroid enlarges = goitre. Cretinism (child), myxoedema (adult).
Insulin (β cells) lowers blood glucose; glucagon (α cells) raises it — an antagonistic pair.
Diabetes mellitus = insulin disorder (high blood sugar, glycosuria); diabetes insipidus = ADH disorder (dilute urine, no sugar).
Adrenal medulla → adrenaline (emergency, fight or flight); adrenal cortex → cortisol and aldosterone.
Calcitonin (thyroid) lowers blood calcium; PTH (parathyroid) raises it — the calcium-balancing antagonistic pair.
Negative feedback holds each hormone at a set-point, which is why BOTH excess and deficiency cause disease.

Practice questions (8)

Attempt each one before opening the answer. Every explanation names the tempting wrong option as well as the right one, because that is where marks are lost.

A dietary deficiency of iodine leads to goitre because low thyroxine causes:
1. The pituitary to over-secrete TSH, enlarging the thyroid
2. The thyroid to make excess thyroxine
3. Blood calcium to rise sharply
4. Insulin secretion to stop
Show answer
Answer: A — The pituitary to over-secrete TSH, enlarging the thyroid
With thyroxine low, the negative feedback brake on the pituitary is released, so TSH rises and drives the thyroid to enlarge even though it cannot make more hormone without iodine — that swelling is goitre. The thyroid cannot make excess thyroxine when its raw material is missing, so the second option contradicts the premise.
Diabetes mellitus, marked by high blood glucose and glucose in the urine, results from a disorder of:
1. Insulin
2. ADH
3. Thyroxine
4. Calcitonin
Show answer
Answer: A — Insulin
High blood sugar spilling into urine is the signature of insufficient or ineffective insulin — diabetes mellitus. ADH deficiency causes diabetes insipidus, a completely different condition with dilute urine and normal sugar, which is the trap the shared name sets.
Insulin and glucagon are described as antagonistic because insulin lowers blood glucose while glucagon:
1. Raises blood glucose by breaking down glycogen
2. Also lowers blood glucose, but more slowly
3. Stores glucose as fat
4. Converts glucose to protein
Show answer
Answer: A — Raises blood glucose by breaking down glycogen
Glucagon from the pancreatic α cells breaks glycogen down to glucose, raising blood sugar — the exact opposite of insulin, which is why the two form an antagonistic pair. An option in which glucagon also lowers glucose removes the antagonism the question is built on.
Oxytocin and ADH, released from the posterior pituitary, are actually synthesised in the:
1. Hypothalamus
2. Anterior pituitary
3. Adrenal cortex
4. Thyroid gland
Show answer
Answer: A — Hypothalamus
The posterior pituitary only stores and releases oxytocin and ADH; both are made by neurons in the hypothalamus and transported down for release. This underlines that the hypothalamus, not the pituitary, is the true controller.
Blood calcium level is kept constant by the antagonistic action of:
1. Calcitonin (lowers) and parathormone (raises)
2. Insulin (lowers) and glucagon (raises)
3. Adrenaline and noradrenaline
4. Oestrogen and progesterone
Show answer
Answer: A — Calcitonin (lowers) and parathormone (raises)
Calcitonin from the thyroid lowers blood calcium and parathormone from the parathyroid raises it, and the balance between them holds calcium steady. Insulin and glucagon do the same job for glucose, not calcium, which is why they are the tempting but wrong pair here.
The "emergency hormone" adrenaline, released during fear or stress, is secreted by the:
1. Adrenal medulla
2. Adrenal cortex
3. Anterior pituitary
4. Pineal gland
Show answer
Answer: A — Adrenal medulla
The adrenal medulla releases adrenaline and noradrenaline for the fight-or-flight response, speeding the heart and raising blood sugar. The adrenal cortex, a separate part of the same gland, makes cortisol and aldosterone instead, so distinguishing medulla from cortex is the point of the question.
The pineal gland secretes melatonin, whose chief role is to regulate:
1. The sleep–wake (circadian) cycle
2. Blood calcium level
3. The rate of digestion
4. Sperm production
Show answer
Answer: A — The sleep–wake (circadian) cycle
Melatonin from the pineal gland governs the daily sleep–wake rhythm and other circadian patterns. Calcium is handled by calcitonin and PTH and sperm by testosterone, so those belong to entirely different glands.
Why do both excess and deficiency of a hormone typically cause disease?
1. The body is tuned to a set-point by negative feedback, so departures either way disrupt it
2. Hormones can never be measured accurately
3. Only excess of a hormone ever causes disease
4. Hormones act only on the gland that makes them
Show answer
Answer: A — The body is tuned to a set-point by negative feedback, so departures either way disrupt it
Negative feedback holds each hormone near an optimal set-point, so moving away from it in either direction — too much or too little — throws the regulated function out of balance and produces a disorder. That is exactly why the disorder table lists a hyper and a hypo condition for each hormone.