Puberty is a chain reaction, and it starts in the brain. One small structure releases one hormone, which triggers a second gland, which triggers the gonads — and only at the end of that chain do any visible changes happen. Learn the chain in order and every question on this page becomes the same question.
📚 What you need to know
During puberty, reproductive hormones cause secondary sexual characteristics to develop.
Two types of sex hormone are involved: gonadotropin-releasing hormone (GnRH) and the steroid sex hormones, oestrogen (also called oestradiol) and testosterone.
The onset of puberty is triggered by the hypothalamus in the brain, which secretes GnRH.
This is detected by the pituitary gland, sitting underneath the hypothalamus, which secretes LH and FSH.
LH and FSH are produced in both females and males, and they enhance the effects of oestrogen and testosterone.
Following this, the secondary sexual characteristics develop.
The cascade
Every stage of puberty runs through the same four-step chain. The hormones travel everywhere in the bloodstream, but each only has an effect on its target organ, which is why a hormone from the brain can act on an organ at the other end of the body and on nothing in between.
The chain to memorise
hypothalamus (GnRH) → pituitary (FSH and LH) → gonads (testosterone or oestrogen) → secondary sexual characteristics
If a question asks what triggers puberty, the answer is the hypothalamus releasing GnRH — not testosterone or oestrogen, which come two steps later.
Students often name testosterone or oestrogen as the trigger for puberty. They are the effectors, not the trigger. The trigger is GnRH from the hypothalamus.
Secondary sexual characteristics
These are the changes that appear once the sex hormones are circulating. They are called secondary because they are not the reproductive organs themselves, which are the primary characteristics present from birth.
Male
Female
Larynx increases in size, deepening the voice
Development of breasts
Increased skeletal muscle development
Widening of the pelvis
Growth of pubic, armpit and body hair, including facial hair
Growth of pubic and armpit hair
Further development and enlargement of the testes, scrotum, penis and glands of the reproductive tract
Enlargement of the vagina and uterus, and maturation of the ovaries
Continuous production of sperm
Monthly ovulation and menstruation
General changes in behaviour associated with an increased sex drive
Deposition of fat under the skin of the buttocks and thighs, and general changes in behaviour associated with an increased sex drive
One shared list, two outcomes. The same four-step cascade runs in both sexes, and both produce FSH and LH. What differs is which gonad responds and therefore which steroid hormone dominates.
Worked examples
WORKED EXAMPLE
Outline the sequence of hormonal events that leads to the development of secondary sexual characteristics at puberty. [4]
Step 1
The hypothalamus in the brain secretes gonadotropin-releasing hormone (GnRH).
Step 2
This is detected by the pituitary gland, which secretes FSH and LH into the blood.
Step 3
FSH and LH act on the gonads — the testes or ovaries — and enhance the effects of the steroid sex hormones testosterone and oestrogen.
Step 4
The increased levels of sex hormones cause the secondary sexual characteristics to develop.
4 marksMarks are given for the order as much as for the names. Keep it as a chain.
WORKED EXAMPLE
FSH and LH are carried in the blood to every organ in the body, yet they only affect the gonads. Explain why. [2]
Point 1 — receptors
A hormone can only affect a cell that has complementary receptors for it.
Point 2 — specificity
Only cells of the target organ, in this case the testes or ovaries, carry those receptors, so the hormone passes other organs without any effect.
2 marksThis receptor argument works for any hormone question in the whole syllabus.
💡 Exam tip
Learn the cascade as four boxes. Almost every question is asking you to reproduce part of it.
The hypothalamus starts it; the pituitary sits underneath it. Getting the two the wrong way round is a common slip.
GnRH stands for gonadotropin-releasing hormone — write it in full at least once.
Oestrogen and testosterone are steroid hormones. FSH, LH and GnRH are not.
Distinguish primary characteristics (present at birth) from secondary ones (developing at puberty).
Use the target organ and receptor argument for any “why only there?” question.
⚠ Common mix-up
Testosterone does not start puberty. GnRH does.
FSH and LH are not female-only hormones. Both sexes produce them.
The pituitary is not part of the hypothalamus. They are separate structures that work together.
Secondary sexual characteristics are not the reproductive organs. Those are primary.
Hormones do not travel to one place. They travel everywhere and act in one place.
Puberty is a process, not an event. The changes develop over years.
Up next: Gametogenesis — how sperm and eggs are actually built, and why one process makes four cells and the other makes one.
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