Low levels of serotonin is real and causes a multitude of illnesses not just depression. Levels of serotonin can be obtained via blood draws. Some of the effects of low serotonin are:
"Serotonin exerts its major effects on the parts of the brain responsible for controlling mood, appetite, sexual desire and performance, sleep, memory, learning, social interactions and temperature regulation.
However, outside the brain, serotonin also affects the cardiovascular system, the endocrine system and muscles. It has even been found to contribute to the regulation of milk production in the breast."
Here is a link about low serotonin its effects and ways to increase the levels.
There are a number of ways to increase your levels of serotonin which include diet and exercise, being in the sun, and medication. To ignore any symptoms of low levels of serotonin, is unwise. I say all this bc many times depression isn't a sign of a "bad" relationship with God. It's the least understood organ in the body. It's also the least testable. I've seen some of my patients who have had an injury to their brain become completely different people both in mood and behavior. The brain uses chemicals and electricity to function and help other organs function. We shouldn't treat it any differently than we would any other major organ.