Testes physiology Flashcards
Feedback loop mechanisms
LH: leydig cells, make testosterone, shuts off LH ß subunit and GnRH. FSH: sertoli cells, make inhibin, shuts off FSH ß subunit.
How does testosterone turn off GnRH?
Aromatases in hypo convert testosterone to estrogen which is the real inhibitor. Whaaaa.
How to measure primary or secondary gonadal failure?
Measure Testosterone and LH. Low T, High LH = primary, Low T, normal/low LH = pituitary failure
Fn of sertoli cells
“nurse” spermatogenesis, make androgen binding protein (carry testosterone from leydig to seminiferous tubules), make inhibin (shut off FSH)
How to measure failing sertoli cells?
Low inhibinm high FSH, small testes. Very common cause of infertility, cause unknown, no Tx.
Role of prolactin and male gonadal fn
Increased prolactin inhibits LH/FSH release, leading to low testosterone/sperm production; Therefore, any cause of increased prolactin can lead to decreased testicular function; Does not participate in general feedback regulation
Fertile eunuch syndrome
Still some sperm but lower FSH, lower T, high LH. Leads to testicular failure.
Androgen transport
Bioavailable: Most on albumin, little unbound. Lots on SHBG (sex hormone binding globulin).
Actions of testosterone and its derivatives
Some converted to DHT: external genitalia, prostate growth, acne, hair, scalp hair loss. Testosterone: wolffian duct, bone formation, muscle, sperm. Little converted to estradiol: hypo shut off, bone resorption, gynecomastia, some other effects.
Testosterone measurement
Free testosterone assay is not so good. Low number of binding proteins, you get artificially low T. Bioactive T assay is best measure of T level. Can be confusing!
Obesity and T levels?
Obesity decreases SHBG so total T level looks low, but it doesn’t affect free T or bioavailable T, so measure the bioavailable.
Why do free hormones not change even when binding proteins do?
Low SHBG would lead to higher free T, but this acts as feeback, so you make less T, ends up at same level of free T. (but lower total T)
