Men produce "approximately 5-15 mg of progesterone in their testicles each day"[1
]. Another 1 mg or so is produced by the adrenals. Because testicular production is tied to testosterone production, if you're shut down by TRT or aging then progesterone production also suffers[2
]. In older men, using hCG may not restore progesterone production[3
The current NIH reference range for serum progesterone in adult males is 0.2 - 1.4 ng/mL[4
]. Lab Corp's standard test has a range of 0.0 - 0.5 ng/mL. It's not clear what level is going to be optimal for any individual. But it's likely that if your value is low or high relative to the reference range then it's not optimal.
There is a sensitive LC/MS-based test for progesterone, but it may not be needed in routine testing, and it's slower and more expensive. There's also a test for free progesterone, which could have the same drawbacks. It's not clear to me how analogous the situation is to that with estradiol. The standard progesterone test does have interference problems with some B vitamins and other things[5
Most doctors may not consider it worthwhile to test progesterone in men. Fortunately it's an inexpensive test if you want to run it yourself. For example, it's currently $23 through Defy Medical.
Regarding high progesterone, the LiveStrong reference says that it can cause an increase in estradiol production, with associated problems. In addition, high progesterone may lead to sleepiness, depression and digestive problems[6
Low progesterone may harm libido and erections. It's potentially linked to prostate and bone density issues, and depression[7
]. Be sure to look at the list of effects of progesterone in this reference
I do believe that progesterone needs more attention. Low levels may be responsible for some of the problems seen in men on TRT who otherwise have good numbers. I've been experimenting with supplementation with good results.