Luteinising hormoneLH
The pituitary hormone that triggers ovulation and testosterone production.
Luteinising hormone comes from the pituitary alongside FSH. In women a mid-cycle surge triggers ovulation; in men it signals the testes to produce testosterone.
It matters because it separates two very different causes of a low sex hormone level. If testosterone is low and LH is high, the signal is arriving but the testes are not responding; if both are low, the problem is higher up, in the pituitary or hypothalamus.
In practice LH is read with FSH and the relevant sex hormone, and in women its interpretation depends entirely on the cycle day the sample was taken.