Luteolin, a testosterone booster from parsley
Luteolin, a flavonoid in parsley and cellery, may increase testosterone production by the testes. Canadian chemists at Moncton University have published an in vitro study in Cell Biology and Toxicology that is of particular interest to the supplement industry.Study
The Canadians exposed MA-10 testes to various flavonoids in test tubes, and then measured their effect on steroid hormone production. The most promising of these was luteolin, a flavonoid similar to quercetin. Therefore, in this post, we will not consider the other flavonoids studied.

And before you ask: luteolin did not damage the cells.
The researchers combined luteolin with forskolin [FSK]. Forskoline mimics the effect of steering hormones like LH, which under normal circumstances stimulate the testes to produce testosterone.
More than the other flavonoids, luteolin increased the production of progesterone in the testes. In the biosynthesis of testosterone, progesterone is a precursor to testosterone. If you want to know more about this, you can take a look at our good old Anabolics Book.

You can see above how luteolin increases the production of testosterone. Luteoline activates the StAR protein, which plays a key role in the conversion of cholesterol into steroid hormones.
"Luteolin effectively increased cAMP-dependent accumulation of progesterone from MA-10 Leydig cells, possibly through activation of Star [...]", the researchers conclude. "Thus, dietary luteolin could be potentially effective to maintain steroidogenesis within aging males."

"However, although there is a correlation between increased StAR [...] and stimulation of steroidogenesis within testicular Leydig cells, more research will be required to clearly define the regulatory mechanism of luteolin on testosterone synthesis."
Luteolin could be an interesting ingredient for testosterone boosters for yet another reason. According to in vitro studies and animal studies, luteolin is also an anti-estrogen...
Cell Biol Toxicol. 2018;34(1):23-38.