Some of these are inter-related. Low adrenals can lead to higher histamine levels. And low thyroid can lead to elevated homocysteine levels.
Also keep in mind that serotonin is produced in two areas of the body for different purposes. The flora generate most of the body's serotonin for things such as intestinal peristalsis and blood pressure control. The brain also generates serotonin for mood, controlling libido, controlling appetite, etc.