24 hour 17-hydroxycorticosteroids (17 ohc/17ohs) is a test that used to be the gold standard back before my time, was all-but thrown out when 24 hr UFC's came along, and is now making a bit of a come-back.
A 17ohc is also a 24 hour urine collection and is collected in exactly the same manner a 24 hour UFC. And...as long as the lab processes it correctly (and neither require preservative at your particular lab), you can also run BOTH a UFC and 17ohc off of one 24 hour urine collection sample (IE, the same 24hr period, the same jug). This makes it a super convenient test to add into the mix.
My understanding is that 17 ohc's are a product of the production/breakdown of cortisol within the body. So, instead of measuring the amount of cortisol your body is passing unused through your kidneys, it is measuring what is more akin to the amount of cortisol your body produced (what is passed into your urine and what was metabolized elsewhere in your body).
Despite having been practically replaced by the 24 hr UFC to the point that labs stopped offering the test and more-recently-trained doctors didn't learn much about it, they are now finding that it is a useful test in helping to confirm cyclic or episodic disease when other test results are mixed (in a situation where some testing comes back high and others come back low/normal).
The results used in this test are the mg/day (as opposed to mg/creat).