Internal Factor, Endogenous Factor
The inside cause is an internal cause. Internal causes are usually the nastiest causes, if you ask me.
This word is mostly used in the fields of medicine and psychology. You’ll often see it as 内因性, which is a technical term used to talk about factors that originate within a person or an organism.
This is a jukugo word, which usually means on'yomi readings from the kanji. If you know the readings of your kanji you'll know how to read this as well.
I was diagnosed with intrinsic asthma.
When a traumatic thing happens, endogenous opioids, which are morphine-like substances, will be secreted.
The real reason I’ve been suffering for a long time is apparently because of an endogenous mental illness.