Inflammation in the gut, and even changes in the composition of the microbiome, can trigger or suppress specific chemicals in the brain. Some of these reactions in the brain can decrease dopamine or serotonin, or other modulators of mood and energy and create states of depression, boredom, anxiety, lassitude, or other mental states and conditions. The opposite is true as well- a balanced, healthy microbiome- with the right mix of good bacteria, can result in healthy and happy mental states.
Many of us think of medical testing ordered by our doctor as something to fear because it often isn’t ordered in conventional medicine offices unless the doctor fears something is wrong. Conventional medicine also tends to have a relatively narrow range of testing they will order on a patient, at least at a primary care doctor. What's more, many of those tests are designed to show only large departures from normal- you either do or don’t have a condition- the reference ranges are designed to quickly pick out those that meet the threshold to be treated by the standard of care and for prescription drugs.