Testosterone and oestradiol
End hormone or mediator · Aromatase 5-alpha reductase SHBG
The gonadal end hormones: theca and Leydig androgen, aromatised in granulosa cells to oestradiol. Both act on nuclear receptors and both feed back on the hypothalamus and gonadotroph.
Traced from the start
Detail
- Amplification
- 5-alpha reductase converts testosterone to dihydrotestosterone in genital skin, prostate and hair follicle — the ligand of external virilisation and of benign prostatic hyperplasia
- Transport
- Both are largely SHBG-bound; SHBG rises with oestrogen and thyrotoxicosis and falls with obesity and insulin resistance, so total testosterone can mislead
- Feedback direction
- Oestradiol is inhibitory at low and moderate levels but becomes stimulatory when high and sustained — the one routine positive loop in this system
- Bone
- Oestradiol, in both sexes, closes the epiphyses and maintains bone density
When it goes wrong
Aromatase deficiency or an oestrogen receptor mutation in a man
The epiphyses never fuse: continued linear growth with osteoporosis, proving that oestrogen rather than testosterone closes the growth plate
Untreated hypogonadism of any cause
Loss of libido, reduced muscle mass, infertility and osteoporosis; in women also vasomotor symptoms and accelerated bone loss
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