Endocrine and reproductive physiology
Introduced| English | Meaning |
|---|---|
| hypothalamus/ˌhaɪpəʊˈθælæməs/ | A brain region that helps regulate homeostasis and links nervous control with hormonal signalling. |
| pituitary/pɪˈtjuːɪtəri/ | The gland below the hypothalamus that releases hormones controlling other glands. |
| target gland | A gland, such as the thyroid or adrenal, driven by a pituitary hormone. |
| primary/ˈpraɪməri/ | Describing endocrine failure of the target gland itself, so its hormone is low and the pituitary's trophic hormone is high. |
| secondary/ˈsekəndəri/ | Describing endocrine failure of the pituitary, so both the trophic hormone and the target gland's hormone are low. |
| menstrual cycle/ˈmenstruːəl ˈsaɪkl/ | The roughly 28-day hormonal cycle of follicle growth, ovulation and endometrial change. |
| ovulation/ˌɒvjʊˈleɪʃn/ | The release of an egg cell from an ovary. |
The slow control layer
- Hormones are the body's slow control layer: they signal along an axis, act at a distance and feed back at the top.
- Every axis follows one grammar, so learning it once explains a dozen syndromes.
The axis
- The hypothalamus 下丘脑 releases a releasing hormone; the pituitary 垂体 answers with a trophic hormone; the target gland 靶腺 makes the effector hormone.
- The effector hormone feeds back negatively on both the pituitary and the hypothalamus.
Put the thyroid axis in order.
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Releasing, trophic and effector hormones, then negative feedback to both higher levels.
Which level failed?
- In primary 原发性 failure the target gland fails: the effector hormone is low and the trophic hormone is HIGH, because feedback is released.
- In secondary 继发性 failure the pituitary fails: both are low. One pattern pair works for every axis.
Low cortisol with a high ACTH points to failure where?
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The pituitary is responding to the low cortisol by raising ACTH, so the gland it drives has failed.
Why TSH is the screen
- TSH responds very steeply to free T4: a small fall in T4 makes TSH rise many-fold.
- So the axis amplifies an early thyroid failure before symptoms or a clearly low T4 appear. Feedback, not assay sensitivity, makes TSH the screening test.
TSH can rise above normal while free T4 is still within its normal range.
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TSH responds very steeply to small T4 changes, so it signals early thyroid failure before T4 leaves its range.
The reproductive axis
- GnRH → LH and FSH → ovaries or testes → sex steroids, with inhibin feeding back on FSH alone. The menstrual cycle 月经周期 is this axis on a monthly rhythm.
- Once per cycle, rising oestradiol flips from negative to positive feedback and triggers the LH surge: ovulation 排卵. The phase after it lasts about 14 days.
The phase after ovulation lasts about 14 days. In a regular 35-day cycle, on about which day does ovulation happen?
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35 − 14 = day 21. Cycle length varies mostly in the phase before ovulation.
What triggers the LH surge?
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Sustained high oestradiol flips the axis's sign once per cycle, releasing a burst of LH that triggers ovulation.
Worked example: reading thyroid labs
- (Fictional labs) Low free T4 with high TSH: the thyroid has failed (primary). The pituitary is shouting at a gland that cannot answer.
- Low free T4 with low or normal TSH: the pituitary or hypothalamus has failed (secondary). Same hormone, opposite partner, different disease.
Match each pattern to the failing level.
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Read each effector hormone with its trophic partner and the level of the fault follows.
Worked example: high LH without ovulation
- (Fictional work-up) No periods and a high LH. Without ovulation there is little progesterone, and progesterone normally slows the hypothalamus's GnRH pulses.
- Faster pulses favour LH over FSH, so LH runs high (the PCOS pattern). Oestrogen without progesterone also explains the risk to the uterine lining.
Three common slips
- Reading one hormone alone. A cortisol or T4 means little without its trophic partner.
- The posterior pituitary as a gland. It stores and releases ADH and oxytocin made in the hypothalamus; it is neural tissue.
- Mixing the cycle's phases. Oestrogen leads before ovulation; progesterone leads after it.
Recap
- Releasing → trophic → effector hormone, with feedback to both higher levels.
- Primary failure: trophic high. Secondary failure: both low. TSH's steep response makes it the screen.
- Ovulation is oestradiol's switch to positive feedback, driving the LH surge.