Biology · 16 Reproduction

16.5 Sex hormones in humans

Before you start

A hormone is a chemical messenger, made in a gland, carried in the blood plasma, and acting on target organs.
The parts of the female reproductive system: ovaries, oviducts, uterus, cervix, vagina — and that an egg travels ovary to oviduct to uterus.
Fertilisation is the fusion of the nuclei of a sperm and an egg cell; implantation is the embryo sinking into the lining of the uterus.
Reading a line graph: identifying a peak, a fall, and the value on a stated day.

Syllabus objectives covered (4)

Describe the roles of testosterone and oestrogen in the development and regulation of secondary sexual characteristics during puberty
Describe the menstrual cycle in terms of changes in the ovaries and in the lining of the uterus
Describe the sites of production of oestrogen and progesterone in the menstrual cycle and in pregnancy
Explain the role of hormones in controlling the menstrual cycle and pregnancy, limited to FSH, LH, progesterone and oestrogen

Part 1 of 7

Puberty: what testosterone and oestrogen actually do

Start with the source organ, because it is often a mark on its own. Testosterone is secreted by the testes. Oestrogen is secreted by the ovaries. Learn that pairing before anything else.
Nothing new appears at puberty. Both hormones are present in small amounts in childhood; at puberty the amount secreted rises sharply, and it is that rise in concentration in the blood that starts the changes.
Testosterone causes the male secondary sexual characteristics: growth of the penis and testes, growth of facial, underarm and pubic hair, deepening of the voice as the larynx enlarges, and increased muscle development. It also starts sperm production in the testes.
Oestrogen causes the female secondary sexual characteristics: development of the breasts, growth of underarm and pubic hair, widening of the hips, and the start of the menstrual cycle, including ovulation.
The word secondary is doing real work in that objective. The primary sexual characteristics are the reproductive organs themselves, present from birth. Secondary characteristics are the features that develop later, at puberty. If the question asks for a secondary sexual characteristic and you write 'has ovaries', you score nothing — that is a primary characteristic.
The objective says 'development and regulation'. Development is the one-off change at puberty. Regulation is what happens afterwards: the hormones keep being secreted, and that continued supply maintains those features and keeps sperm production or the menstrual cycle running. Take the hormone away and the maintained features fade.

Part 2 of 7

The menstrual cycle, seen in the lining of the uterus

The cycle is a loop, so someone has to decide where it starts. By convention, day 1 is the first day of bleeding. Every graph and every exam question numbers days from that point, so getting this right decides whether you read the rest correctly.
Days 1 to about 5 — menstruation. The lining of the uterus breaks down and is lost through the vagina, together with some blood. This is the lining leaving, not an egg leaving.
Days 5 to 14 — the lining repairs itself and grows thicker, developing a rich supply of blood vessels. It is being prepared so that an embryo could sink into it and be supplied with food and oxygen.
Day 14 in a 28-day cycle — ovulation. Notice that this is not a uterus event at all; it happens in the ovary. Keep the two organs in two separate columns in your head.
Days 14 to 28 — the lining is maintained: it stays thick and well supplied with blood, ready for implantation. Nothing dramatic happens here, and that is the point; something is actively holding it in place.
If no embryo implants, at about day 28 the lining breaks down again — which is day 1 of the next cycle. The loop has closed.
28 days is an average, and it is the number exam questions use unless they tell you otherwise. Real cycles vary in length. If a question gives you a cycle length, work with theirs, not with 28.

Part 3 of 7

The same cycle, seen in the ovary

In the first half of the cycle, a follicle matures in one ovary. A follicle is a ball of cells with an immature egg cell inside it. The egg cell was already there; what the cycle does is mature one of them.
At ovulation the mature follicle bursts and the egg is released from the ovary into the oviduct. Say 'released from the ovary'. 'Released from the uterus' is a common slip and it is simply wrong.
What is left of the follicle does not just vanish. It develops into the corpus luteum, a temporary yellow structure that stays in the ovary for the rest of the cycle. Remember it exists, because it turns out to be the thing that keeps the whole second half going.
If the egg is not fertilised, the corpus luteum breaks down over the last few days of the cycle. That collapse is what ends the cycle — you will see exactly why once the hormones are added.
So two organs change at the same time. The objective says 'in the ovaries and in the lining of the uterus'. An answer that describes only the bleeding and the thickening has answered half the question and can only get half the marks.

Part 4 of 7

Where oestrogen and progesterone are produced

During the cycle, oestrogen is secreted by the ovary — specifically by the developing follicle. Its concentration climbs through the first half as the follicle grows.
During the cycle, progesterone is secreted by the corpus luteum, which is also in the ovary. The corpus luteum only exists after ovulation, so progesterone is essentially a second-half hormone. Before day 14 there is very little of it, and now you know why: there is nothing there to make it.
In pregnancy the site changes. For the first weeks the corpus luteum survives and keeps secreting progesterone. Then the placenta takes over, and from roughly the third month onwards the placenta is the site of production of both progesterone and oestrogen.
So 'where is it made?' has a different answer depending on when. Hold three lines: follicle in the ovary makes oestrogen during the cycle; corpus luteum in the ovary makes progesterone after ovulation and in early pregnancy; placenta makes both in the later part of pregnancy.
When a question asks for the site of production, name a structure — follicle, corpus luteum, placenta — or at least the organ. 'In the body' or 'in the blood' earns nothing; the blood is where a hormone is transported, not where it is made.

Part 5 of 7

The four hormones, in the order they act

The syllabus limits this to four hormones: FSH and LH, secreted by the pituitary gland in the head, and oestrogen and progesterone, secreted by the ovary. Two from the brain, two from the ovary — that split is the shape of the whole answer, and it is why the two sets can control each other.
FSH (follicle stimulating hormone) is released by the pituitary early in the cycle. It travels in the blood to the ovary and does two things: it stimulates a follicle to mature, and it stimulates the ovary to secrete oestrogen.
Be precise here. FSH does not make eggs. The egg cells are already in the ovary; FSH causes one follicle, with its egg, to mature. 'FSH produces the egg' loses the mark.
Oestrogen then has two effects, one in each direction. In the uterus, it stimulates the lining to repair and thicken. Back at the pituitary, it inhibits FSH — so no more follicles start maturing — and it stimulates the release of LH.
LH (luteinising hormone) rises to a sharp peak at about day 14, and that surge causes ovulation. LH also stimulates what is left of the follicle to become the corpus luteum, which then secretes progesterone.
Progesterone maintains the thick lining of the uterus so it is ready to receive an embryo. It also inhibits FSH and LH, which is why a second follicle does not mature and a second ovulation does not happen in the same cycle.
Now close the loop. If the egg is not fertilised, the corpus luteum breaks down, so the progesterone concentration falls. Nothing is maintaining the lining any more, so the lining breaks down — menstruation. And because progesterone was the thing inhibiting FSH, the fall in progesterone lets FSH rise again and start the next cycle.
That last sentence is where the marks are. Most candidates can say 'progesterone falls'. Fewer add 'so the lining is no longer maintained and breaks down' and 'FSH is no longer inhibited, so a new follicle can mature'. Write all three and the answer is complete.
One word pair to keep straight: oestrogen repairs and thickens the lining; progesterone maintains it. Swap those verbs and you lose the mark even though you named the right hormone.

Part 6 of 7

Pregnancy: why the cycle stops

If fertilisation and implantation happen, the corpus luteum does not break down. It carries on secreting progesterone, so the progesterone concentration stays high instead of falling at the end of the cycle.
High progesterone maintains the lining of the uterus, so it does not break down and menstruation does not happen. That is the direct reason a missed period is the first sign of pregnancy.
High progesterone also continues to inhibit FSH and LH. No FSH means no follicle matures; no LH surge means no ovulation. So no further eggs are released for the whole of the pregnancy.
From about the third month the placenta takes over secretion of progesterone, and oestrogen with it. The source has changed but the effects are the same, so the lining stays intact and ovulation stays switched off.
Stay inside the four named hormones. The objective says 'limited to FSH, LH, progesterone and oestrogen', so a complete answer never needs anything else — and adding a hormone you half-remember risks contradicting a correct point you have already made.

Part 7 of 7

Reading a hormone-level graph

Almost every exam question on this topic gives you hormone concentration plotted against day of the cycle, with the curves unlabelled or labelled A to D. Identify them by shape and by what you know happens, not by trying to remember a picture.
The curve with one sharp, narrow spike at about day 14 is LH. Nothing else in this cycle spikes like that, and the spike sits exactly where ovulation is.
The curve that is low for the whole first half and then forms a broad hump in the second half, falling near the end, is progesterone — because the corpus luteum that makes it only exists in the second half.
The curve that climbs through the first half to a peak just before the LH spike is oestrogen, made by the growing follicle. FSH is the one that is highest early in the cycle and is suppressed later.
Then the standard follow-up: 'Describe what is happening to the lining of the uterus on day X.' Answer it from the progesterone curve. Progesterone high means lining maintained; progesterone falling means the lining is about to break down.

Worked examples

Example 1. A woman has a regular 28-day menstrual cycle. Her blood progesterone concentration is very low on day 10, rises from day 15, reaches its highest value on day 21 and falls steeply between day 25 and day 28. Explain what is happening in her ovary and in her uterus over this time.

Show the working

Take the low value on day 10 first, and ask what makes progesterone. It is the corpus luteum — and on day 10 there is no corpus luteum yet, because ovulation has not happened. Low progesterone before day 14 is expected, not a problem.
Now the rise from day 15. Ovulation happened at about day 14, the follicle became the corpus luteum, and the corpus luteum secretes progesterone. The rise dates the ovulation for you.
Ask what the high progesterone from day 15 to day 25 is doing in the uterus. It maintains the thickened lining, with its rich blood supply, ready for an embryo to implant.
Now the steep fall after day 25. Progesterone can only fall if the structure making it is failing, so the corpus luteum must be breaking down. It does that when the egg has not been fertilised.
Finish with the consequence in the uterus, because that is what the falling curve predicts. With progesterone low, the lining is no longer maintained, so it breaks down and menstruation begins — day 1 of the next cycle.

Answer: Ovulation occurred at about day 14; the follicle became the corpus luteum, which secreted progesterone from day 15 and maintained the thick, well-supplied lining of the uterus. The egg was not fertilised, so the corpus luteum broke down after about day 25, progesterone fell, the lining was no longer maintained and it broke down as menstruation.

Example 2. A different woman's progesterone concentration does not fall at the end of her cycle; it stays high and rises further over the following weeks, and she has no menstruation. Explain what this tells you, and state where the progesterone is being produced at 4 weeks and at 6 months.

Show the working

Start from what makes progesterone fall in a normal cycle: the corpus luteum breaking down. If progesterone has not fallen, the corpus luteum has not broken down.
Ask what stops it breaking down. Fertilisation followed by implantation — so this pattern indicates pregnancy.
Now link progesterone to the missing menstruation. Progesterone maintains the lining of the uterus, so while it stays high the lining is not lost, and the embryo is not lost with it.
For the sites, use the stage. At 4 weeks the corpus luteum in the ovary is still the source.
By 6 months the placenta has taken over as the site of production of progesterone, and of oestrogen. Name the structure, not just 'the mother'.

Answer: She is pregnant: the corpus luteum has not broken down, so progesterone stays high and maintains the lining of the uterus, so menstruation does not occur. At 4 weeks the progesterone is produced by the corpus luteum in the ovary; at 6 months it is produced by the placenta.

Example 3. A 15-year-old boy is investigated because puberty has not started. His blood testosterone concentration is found to be far lower than expected for his age. Suggest three changes that would not yet have taken place in his body, and name the organ that should be secreting the hormone.

Show the working

Begin with the organ, because it is the easy mark: testosterone is secreted by the testes.
Now work from the rule 'the hormone causes the change'. If the hormone concentration is low, the changes it causes will not have been stimulated.
Pick changes from the secondary sexual characteristics list, not from general growing up. Facial and body hair, deepening of the voice as the larynx enlarges, and increased muscle development are all caused by testosterone.
You can also use sperm production, since testosterone starts it. Avoid answering 'he would not have testes' — the testes are a primary sexual characteristic, present from birth, so that is not a change at puberty.

Answer: The testes secrete testosterone. Without it, he would not have grown facial or body hair, his voice would not have deepened (the larynx would not have enlarged) and his muscle development would not have increased; sperm production would also not have started.

Where people go wrong

Mistake: Writing that the egg is released from the uterus, or that menstruation is the egg leaving the body.

Instead: The egg is released from the ovary at ovulation. Menstruation is the breakdown and loss of the lining of the uterus.

They are two different events in two different organs, about two weeks apart. Examiners treat 'egg released from the uterus' as a wrong statement, and if it contradicts a correct statement elsewhere in the same answer, neither can be credited.

Mistake: Saying oestrogen maintains the lining of the uterus.

Instead: Oestrogen causes the lining to repair and thicken in the first half; progesterone maintains it in the second half.

The verb is the marking point. Naming the right hormone with the wrong action does not score, and this is the single most common way marks are lost in this topic.

Mistake: Writing that FSH produces the egg.

Instead: FSH stimulates a follicle, containing an egg cell that is already present, to mature — and stimulates the ovary to secrete oestrogen.

'Stimulates' is the load-bearing word. A hormone is a messenger that causes an existing structure to do something; it does not build cells itself.

Mistake: Explaining menstruation as 'progesterone drops' and stopping there.

Instead: Say that the corpus luteum breaks down, so progesterone falls, so the lining is no longer maintained and breaks down.

The question is usually an Explain, worth two or three marks, and each link in that chain is a separate marking point. A single statement can only ever collect one of them.

Mistake: Answering 'where is progesterone produced?' with 'the ovary' regardless of the stage.

Instead: Corpus luteum (in the ovary) after ovulation and in early pregnancy; placenta in the later stages of pregnancy.

The objective says 'in the menstrual cycle and in pregnancy', so the question is nearly always testing whether you know the source changes.

Check yourself

Name the organ that secretes oestrogen and give two secondary sexual characteristics it causes at puberty.
Answer
The ovaries. Any two of: development of the breasts, growth of underarm and pubic hair, widening of the hips, start of the menstrual cycle including ovulation.
Why is 'having testes' not an acceptable answer for a secondary sexual characteristic?
Answer
The testes are a primary sexual characteristic — a reproductive organ present from birth. Secondary sexual characteristics are the features that develop later, at puberty, such as facial hair and a deepened voice.
What is happening in the ovary and in the uterus on day 8 of a 28-day cycle?
Answer
In the ovary a follicle is maturing. In the uterus the lining is repairing and thickening, developing a rich blood supply.
What happens to the follicle after ovulation, and why does it matter?
Answer
It becomes the corpus luteum, which secretes progesterone. It matters because progesterone is what maintains the lining of the uterus for the second half of the cycle.
Where is oestrogen produced during the menstrual cycle, and where in the later stages of pregnancy?
Answer
During the cycle, by the follicle in the ovary. In the later stages of pregnancy, by the placenta.
State the two effects of oestrogen on the pituitary gland.
Answer
It inhibits the release of FSH, and it stimulates the release of LH.
A hormone graph shows one narrow spike on day 14. Which hormone is it, and what does the spike cause?
Answer
LH. The surge causes ovulation — the release of the egg from the ovary.
Why does no ovulation occur during pregnancy?
Answer
Progesterone (with oestrogen) stays high and inhibits FSH and LH, so no follicle matures and there is no LH surge to trigger ovulation.

Exam questions

Try each part first. Open the mark scheme after you have written your answer.

Question 1.

(a) State the organ that secretes testosterone and the organ that secretes oestrogen. 2 marks No calculator

Mark scheme

B1 testosterone — testes
B1 oestrogen — ovaries / ovary

(b) Describe two secondary sexual characteristics that develop in a boy at puberty as a result of testosterone. 2 marks No calculator

Mark scheme

B1 any one of: growth of facial / body / pubic hair; deepening of the voice / enlargement of the larynx; increased muscle development; growth of the penis and testes; start of sperm production
B1 a second, different characteristic from the same list. Ignore general growth in height. 'Has testes' is a primary characteristic and does not score

Question 2. A woman has a regular 28-day menstrual cycle. Day 1 is the first day of menstruation.

(a) Describe the changes that take place in the lining of the uterus between day 1 and day 14. 3 marks No calculator

Mark scheme

B1 the lining breaks down and is lost / menstruation, days 1 to about 5
B1 the lining then repairs / regrows / thickens
B1 it develops a rich blood supply, ready to receive an embryo / for implantation

(b) Explain the role of LH in this cycle. 3 marks No calculator

Mark scheme

B1 secreted / released by the pituitary gland
B1 rises sharply / peaks at about day 14 and causes ovulation / release of the egg from the ovary
B1 stimulates the remains of the follicle to form the corpus luteum, which secretes progesterone

(c) The egg released in this cycle is not fertilised. Explain why the lining of the uterus breaks down at the end of the cycle. 3 marks No calculator

Mark scheme

B1 the corpus luteum breaks down / degenerates
B1 so the concentration of progesterone falls
B1 progesterone maintains the lining, so the lining is no longer maintained and breaks down. Accept 'FSH is no longer inhibited so the next cycle starts' as an alternative third point

Question 3. A woman becomes pregnant. Her blood progesterone concentration does not fall at the end of the cycle in which fertilisation occurred, and it remains high throughout the pregnancy.

(a) State where progesterone is produced in the second half of a normal menstrual cycle, and where it is produced in the later stages of pregnancy. 2 marks No calculator

Mark scheme

B1 corpus luteum / yellow body, in the ovary
B1 placenta

(b) Explain why menstruation does not occur during pregnancy. 3 marks No calculator

Mark scheme

B1 the corpus luteum does not break down / the placenta secretes progesterone
B1 so the concentration of progesterone remains high
B1 progesterone maintains the lining of the uterus, so it does not break down and the embryo is not lost

(c) Explain why no further eggs are released during the pregnancy. 2 marks No calculator

Mark scheme

B1 high progesterone (and oestrogen) inhibits FSH, so no follicle matures
B1 and inhibits LH, so there is no LH surge and no ovulation

How this is examined

The chain from corpus luteum to falling progesterone to menstruation: a three-mark Explain, usually attached to a graph of hormone concentration against day, asking why the lining of the uterus breaks down at the end of the cycle Three separate links, each written as its own statement: 'the egg is not fertilised so the corpus luteum breaks down'; 'so the concentration of progesterone falls'; 'progesterone maintains the lining of the uterus, so the lining is no longer maintained and breaks down'. Adding 'and FSH is no longer inhibited, so the next cycle begins' secures the third mark if one of the others is judged vague.
What each of FSH, LH, oestrogen and progesterone does, and where it comes from: a four- to six-mark Explain of how the menstrual cycle is controlled, or a table to complete with hormone, source and effect One marking point per hormone, each naming source and action: 'FSH from the pituitary stimulates a follicle to mature and the ovary to secrete oestrogen'; 'oestrogen causes the lining of the uterus to repair and thicken, inhibits FSH and stimulates LH'; 'LH surges at about day 14 and causes ovulation'; 'progesterone from the corpus luteum maintains the lining and inhibits FSH and LH'. The verbs stimulate, inhibit, cause and maintain are what score — swapping them loses the mark.
The cycle described in both organs at once: a three- or four-mark Describe of the changes in the ovaries and in the lining of the uterus over a stated range of days Ovary points and uterus points are marked separately, so give both: 'a follicle matures in the ovary'; 'the egg is released from the ovary at about day 14'; 'the follicle becomes the corpus luteum'; and 'the lining breaks down and is lost days 1 to 5'; 'the lining repairs and thickens'; 'the lining is maintained after ovulation'. Days quoted correctly strengthen a borderline answer.
Why menstruation and ovulation both stop during pregnancy: a two- or three-mark Explain, often following a graph in which progesterone does not fall at day 28 'The corpus luteum does not break down, so progesterone stays high'; 'progesterone maintains the lining of the uterus so it is not lost'; 'progesterone inhibits FSH and LH so no follicle matures and no ovulation occurs'.
The site of production changing between cycle and pregnancy: a one- or two-mark State, asking where oestrogen or progesterone is produced at a stated stage A named structure: follicle (in the ovary) for oestrogen during the cycle; corpus luteum (in the ovary) for progesterone after ovulation and in early pregnancy; placenta for both in the later stages of pregnancy. 'In the blood' scores nothing — that is transport, not production.
Which changes at puberty count as secondary sexual characteristics: a two-mark Describe or State of the role of testosterone or oestrogen, sometimes as a Suggest about a person whose hormone concentration is unusually low The source organ (testes or ovaries) plus named characteristics from the list — facial and body hair, deepened voice, increased muscle development, sperm production; breast development, wider hips, pubic and underarm hair, start of the menstrual cycle. Naming a reproductive organ itself scores nothing, because that is a primary characteristic.

Words to know

Secondary sexual characteristic: A feature that develops at puberty under the control of testosterone or oestrogen, such as facial hair or breast development. Not the reproductive organs themselves, which are present from birth.
Menstruation: The breakdown and loss of the lining of the uterus, with some blood, through the vagina. Day 1 of the cycle is the first day of menstruation.
Ovulation: The release of an egg cell from the ovary, at about day 14 of a 28-day cycle, caused by a surge of LH.
Follicle: A ball of cells in the ovary containing an immature egg cell. FSH stimulates one to mature; it secretes oestrogen.
Corpus luteum: The structure that the follicle becomes after ovulation. It secretes progesterone, and breaks down if the egg is not fertilised.
Inhibit: To reduce or stop the release of something. Oestrogen inhibits FSH; progesterone inhibits both FSH and LH.
Placenta: The organ that develops in the uterus during pregnancy and, from about the third month, becomes the site of production of progesterone and oestrogen.

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